c/100522 - avoid invalid GIMPLE in GIMPLE parsing

This plugs a few easy holes avoiding ICEs down the route.

2021-05-18  Richard Biener  <rguenther@suse.de>

	PR c/100522
gcc/c/
	* gimple-parser.c (c_parser_gimple_postfix_expression_after_primary):
	Diagnose calls to non-functions.
	(c_parser_gimple_statement): Diagnose unexpected assignment RHS.

gcc/testsuite/
	* gcc.dg/gimplefe-error-10.c: New testcase.