c++/modules: #include <vector> -> import <bits/stdc++.h>

Since the standard library doesn't preclude an #include of a standard
library header from bringing in declarations from other headers, we can
translate an #include of any of the importable headers as an import of
<bits/stdc++.h>.

To reduce the amount of C++ standard knowledge encoded in libcpp, I extend
the translate_include callback to allow it to suggest an alternate header to
try translating.  It's a bit awkward to bounce back and forth, but this
seems like the right division of responsibilities.

libcpp/ChangeLog:

	* include/cpplib.h (struct cpp_callbacks): Replace 'path' parameter
	with file, angle_brackets, and alternate name.
	(cpp_get_name): Declare.
	* files.cc (cpp_get_name): New.
	(_cpp_stack_include, _cpp_post_stack_file, _cpp_stack_file)
	(_cpp_stack_translated_file): Refactor, try alternate file.

gcc/cp/ChangeLog:

	* module.cc (maybe_translate_include): Suggest <bits/stdc++.h>
	as an alternate for importable standard library headers.
	(importable_headers, is_importable_header): New.

gcc/ChangeLog:

	* doc/invoke.texi (C++ Modules): Remove standard library header
	units from missing pieces, mention importable header redirection.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/compile-std1.C: Test <vector> translation.

libcpp/include/cpplib.h

@@ -860,7 +860,8 @@ struct cpp_callbacks
   /* Maybe translate a #include into something else.  Return a
      cpp_buffer containing the translation if translating.  */
   char *(*translate_include) (cpp_reader *, line_maps *, location_t,
-			      const char *path);
+			      _cpp_file *file, bool angle_brackets,
+			      const char **alternate);
 };
 
 #ifdef VMS