p1689r5: initial support

This patch implements support for [P1689R5][] to communicate to a build
system the C++20 module dependencies to build systems so that they may
build `.gcm` files in the proper order.

Support is communicated through the following three new flags:

- `-fdeps-format=` specifies the format for the output. Currently named
  `p1689r5`.

- `-fdeps-file=` specifies the path to the file to write the format to.

- `-fdeps-target=` specifies the `.o` that will be written for the TU
  that is scanned. This is required so that the build system can
  correlate the dependency output with the actual compilation that will
  occur.

CMake supports this format as of 17 Jun 2022 (to be part of 3.25.0)
using an experimental feature selection (to allow for future usage
evolution without committing to how it works today). While it remains
experimental, docs may be found in CMake's documentation for
experimental features.

Future work may include using this format for Fortran module
dependencies as well, however this is still pending work.

[P1689R5]: https://isocpp.org/files/papers/P1689R5.html
[cmake-experimental]: https://gitlab.kitware.com/cmake/cmake/-/blob/master/Help/dev/experimental.rst

TODO:

- header-unit information fields

Header units (including the standard library headers) are 100%
unsupported right now because the `-E` mechanism wants to import their
BMIs. A new mode (i.e., something more workable than existing `-E`
behavior) that mocks up header units as if they were imported purely
from their path and content would be required.

- non-utf8 paths

The current standard says that paths that are not unambiguously
represented using UTF-8 are not supported (because these cases are rare
and the extra complication is not worth it at this time). Future
versions of the format might have ways of encoding non-UTF-8 paths. For
now, this patch just doesn't support non-UTF-8 paths (ignoring the
"unambiguously representable in UTF-8" case).

- figure out why junk gets placed at the end of the file

Sometimes it seems like the file gets a lot of `NUL` bytes appended to
it. It happens rarely and seems to be the result of some
`ftruncate`-style call which results in extra padding in the contents.
Noting it here as an observation at least.

libcpp/

	* include/cpplib.h: Add cpp_fdeps_format enum.
	(cpp_options): Add fdeps_format field
	(cpp_finish): Add structured dependency fdeps_stream parameter.
	* include/mkdeps.h (deps_add_module_target): Add flag for
	whether a module is exported or not.
	(fdeps_add_target): Add function.
	(deps_write_p1689r5): Add function.
	* init.cc (cpp_finish): Add new preprocessor parameter used for C++
	module tracking.
	* mkdeps.cc (mkdeps): Implement P1689R5 output.

gcc/

	* doc/invoke.texi: Document -fdeps-format=, -fdeps-file=, and
	-fdeps-target= flags.
	* gcc.cc: add defaults for -fdeps-target= and -fdeps-file= when
	only -fdeps-format= is specified.
	* json.h: Add a TODO item to refactor out to share with
	`libcpp/mkdeps.cc`.

gcc/c-family/

	* c-opts.cc (c_common_handle_option): Add fdeps_file variable and
	-fdeps-format=, -fdeps-file=, and -fdeps-target= parsing.
	* c.opt: Add -fdeps-format=, -fdeps-file=, and -fdeps-target=
	flags.

gcc/cp/

	* module.cc (preprocessed_module): Pass whether the module is
	exported to dependency tracking.

gcc/testsuite/

	* g++.dg/modules/depflags-f-MD.C: New test.
	* g++.dg/modules/depflags-f.C: New test.
	* g++.dg/modules/depflags-fi.C: New test.
	* g++.dg/modules/depflags-fj-MD.C: New test.
	* g++.dg/modules/depflags-fj.C: New test.
	* g++.dg/modules/depflags-fjo-MD.C: New test.
	* g++.dg/modules/depflags-fjo.C: New test.
	* g++.dg/modules/depflags-fo-MD.C: New test.
	* g++.dg/modules/depflags-fo.C: New test.
	* g++.dg/modules/depflags-j-MD.C: New test.
	* g++.dg/modules/depflags-j.C: New test.
	* g++.dg/modules/depflags-jo-MD.C: New test.
	* g++.dg/modules/depflags-jo.C: New test.
	* g++.dg/modules/depflags-o-MD.C: New test.
	* g++.dg/modules/depflags-o.C: New test.
	* g++.dg/modules/p1689-1.C: New test.
	* g++.dg/modules/p1689-1.exp.ddi: New test expectation.
	* g++.dg/modules/p1689-2.C: New test.
	* g++.dg/modules/p1689-2.exp.ddi: New test expectation.
	* g++.dg/modules/p1689-3.C: New test.
	* g++.dg/modules/p1689-3.exp.ddi: New test expectation.
	* g++.dg/modules/p1689-4.C: New test.
	* g++.dg/modules/p1689-4.exp.ddi: New test expectation.
	* g++.dg/modules/p1689-5.C: New test.
	* g++.dg/modules/p1689-5.exp.ddi: New test expectation.
	* g++.dg/modules/modules.exp: Load new P1689 library routines.
	* g++.dg/modules/test-p1689.py: New tool for validating P1689 output.
	* lib/modules.exp: Support for validating P1689 outputs.

Signed-off-by: Ben Boeckel <ben.boeckel@kitware.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

libcpp/include/mkdeps.h

@@ -53,11 +53,15 @@ extern void deps_add_default_target (class mkdeps *, const char *);
 
 /* Adds a module target.  The module name and cmi name are copied.  */
 extern void deps_add_module_target (struct mkdeps *, const char *module,
-				    const char *cmi, bool is_header);
+				    const char *cmi, bool is_header,
+				    bool is_exported);
 
 /* Adds a module dependency.  The module name is copied.  */
 extern void deps_add_module_dep (struct mkdeps *, const char *module);
 
+/* Add a structured dependency target.  */
+extern void fdeps_add_target (struct mkdeps *, const char *, bool);
+
 /* Add a dependency (appears on the right side of the colon) to the
    deps list.  Dependencies will be printed in the order that they
    were entered with this function.  By convention, the first
@@ -68,6 +72,9 @@ extern void deps_add_dep (class mkdeps *, const char *);
    is the number of columns to word-wrap at (0 means don't wrap).  */
 extern void deps_write (const cpp_reader *, FILE *, unsigned int);
 
+/* Write out a deps buffer to a specified file in P1689R5 format.  */
+extern void deps_write_p1689r5 (const struct mkdeps *, FILE *);
+
 /* Write out a deps buffer to a file, in a form that can be read back
    with deps_restore.  Returns nonzero on error, in which case the
    error number will be in errno.  */