c++: Fix location for -Wunused-macros [PR66290]

c++: Fix location for -Wunused-macros [PR66290]

In C++, since all tokens are lexed from libcpp up front, diagnostics generated
by libcpp after lexing has completed do not get a valid location from libcpp
(rather, libcpp thinks they all pertain to the end of the file.) This has long
been addressed using the global variable "done_lexing", which the C++ frontend
sets at the appropriate time; when done_lexing is true, then c_cpp_diagnostic(),
which outputs libcpp's diagnostics, uses input_location instead of the wrong
libcpp location. The C++ frontend arranges that input_location will point to the
token it is currently processing, so this generally works fine. However, there
is one exception currently, which is -Wunused-macros. This gets generated at the
end of processing in cpp_finish (), since we need to wait until then to
determine whether a macro was eventually used or not. But the locations it
passes to c_cpp_diagnostic () were remembered from the original lexing and hence
they should not be overridden with input_location, which is now the one
incorrectly pointing to the end of the file.

Fixed by setting done_lexing=false again just prior to calling cpp_finish (). I
also renamed the variable from done_lexing to "override_libcpp_locations", since
it's now not strictly about lexing anymore.

There is no new testcase with this patch, since we already had an xfailed
testcase which is now fixed.

gcc/c-family/ChangeLog:

	PR c++/66290
	* c-common.h: Rename global done_lexing to
	override_libcpp_locations.
	* c-common.cc (c_cpp_diagnostic): Likewise.
	* c-opts.cc (c_common_finish): Set override_libcpp_locations
	(formerly done_lexing) immediately prior to calling cpp_finish ().

gcc/cp/ChangeLog:

	PR c++/66290
	* parser.cc (cp_lexer_new_main): Rename global done_lexing to
	override_libcpp_locations.

gcc/testsuite/ChangeLog:

	PR c++/66290
	* c-c++-common/pragma-diag-15.c: Remove xfail for C++.