input: give file_cache_slot its own copy of the file path [PR118919]

input: give file_cache_slot its own copy of the file path [PR118919]

input.cc's file_cache was borrowing copies of the file name.
This could lead to use-after-free when writing out sarif output
from Fortran, which frees its filenames before the sarif output
is fully written out.

Fix by taking a copy in file_cache_slot.

gcc/ChangeLog:
	PR other/118919
	* input.cc (file_cache_slot::m_file_path): Make non-const.
	(file_cache_slot::evict): Free m_file_path.
	(file_cache_slot::create): Store a copy of file_path if non-null.
	(file_cache_slot::~file_cache_slot): Free m_file_path.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>