c-family: Improve location for -Wunknown-pragmas in a _Pragma [PR118838]

The warning for -Wunknown-pragmas is issued at the location provided by
libcpp to the def_pragma() callback. This location is
cpp_reader::directive_line, which is a location for the start of the line
only; it is also not a valid location in case the unknown pragma was lexed
from a _Pragma string. These factors make it impossible to suppress
-Wunknown-pragmas via _Pragma("GCC diagnostic...") directives on the same
source line, as in the PR and the test case. Address that by issuing the
warning at a better location returned by cpp_get_diagnostic_override_loc().
libcpp already maintains this location to handle _Pragma-related diagnostics
internally; it was needed also to make a publicly accessible version of it.

gcc/c-family/ChangeLog:

	PR c/118838
	* c-lex.cc (cb_def_pragma): Call cpp_get_diagnostic_override_loc()
	to get a valid location at which to issue -Wunknown-pragmas, in case
	it was triggered from a _Pragma.

libcpp/ChangeLog:

	PR c/118838
	* errors.cc (cpp_get_diagnostic_override_loc): New function.
	* include/cpplib.h (cpp_get_diagnostic_override_loc): Declare.

gcc/testsuite/ChangeLog:

	PR c/118838
	* c-c++-common/cpp/pragma-diagnostic-loc-2.c: New test.
	* g++.dg/gomp/macro-4.C: Adjust expected output.
	* gcc.dg/gomp/macro-4.c: Likewise.
	* gcc.dg/cpp/Wunknown-pragmas-1.c: Likewise.

libcpp/include/cpplib.h

@@ -1169,6 +1169,8 @@ extern const char *cpp_probe_header_unit (cpp_reader *, const char *file,
 extern const char *cpp_get_narrow_charset_name (cpp_reader *) ATTRIBUTE_PURE;
 extern const char *cpp_get_wide_charset_name (cpp_reader *) ATTRIBUTE_PURE;
 
+extern location_t cpp_get_diagnostic_override_loc (const cpp_reader *);
+
 /* This function reads the file, but does not start preprocessing.  It
    returns the name of the original file; this is the same as the
    input file, except for preprocessed input.  This will generate at