c: _Generic should not warn in non-active branches [PR68193,PR97100,PR110703]

To avoid false diagnostics, use c_inhibit_evaluation_warnings when
a generic association is known to not match during parsing.  We may
still generate false positives if the default branch comes earler than
a specific association that matches.

PR c/68193
PR c/97100
PR c/110703

gcc/c/:
	* c-parser.cc (c_parser_generic_selection): Inhibit evaluation
	warnings branches that are known not be taken during parsing.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr68193.c: New test.