C: Avoid incorrect warning for volatile in compound expressions [PR98260]

2020-12-16  Martin Uecker  <muecker@gwdg.de>

gcc/c/
	PR c/98260
	* c-parser.c (c_parser_expression): Look into
	nop expression when marking expressions as read.

gcc/testsuite/
	PR c/98260
	* gcc.dg/unused-9.c: New test.