Selftests can use class selftest::temp_source_file to write out files
for testing input-handling, and the files are unlinked in the dtor.

This leads to stale entries in input.c's cache of file content, which
could lead to errors if a temporary filename gets reused during a run
of the selftests.

We don't normally expect files to be "deleted from under us", so
special-case this by adding a special way for temp_source_file's
dtor to purge any cache entries referring to it.

gcc/ChangeLog:
	* input.c (diagnostics_file_cache_forcibly_evict_file): New
	function.
	* input.h (diagnostics_file_cache_forcibly_evict_file): New
	declaration.
	* selftest.c (selftest::temp_source_file::~temp_source_file):
	Evict m_filename from the diagnostic file cache.

From-SVN: r239570