c: Fix -Wunused-but-set-* warning with _Generic [PR96571]

c: Fix -Wunused-but-set-* warning with _Generic [PR96571]

The following testcase shows various problems with -Wunused-but-set*
warnings and _Generic construct.  I think it is best to treat the selector
and the ignored expressions as (potentially) read, because when they are
parsed, the vars in there are already marked as TREE_USED.

2020-08-18  Jakub Jelinek  <jakub@redhat.com>

	PR c/96571
	* c-parser.c (c_parser_generic_selection): Change match_found from bool
	to int, holding index of the match.  Call mark_exp_read on the selector
	expression and on expressions other than the selected one.

	* gcc.dg/Wunused-var-4.c: New test.

(cherry picked from commit 6d42cbe5ad7a7b46437f2576c9920e44dc14b386)