c/118742 - gimple FE parsing of unary operators of C promoted args

The GIMPLE FE currently invokes parser_build_unary_op to build
unary GENERIC which has the operand subject to C promotion rules
which does not match GIMPLE.  The following adds a wrapper around
the build_unary_op worker which conveniently has an argument to
indicate whether to skip such promotion.

	PR c/118742
gcc/c/
	* gimple-parser.cc (gimple_parser_build_unary_op): New
	wrapper around build_unary_op.
	(c_parser_gimple_unary_expression): Use it.

gcc/testsuite/
	* gcc.dg/gimplefe-56.c: New testcase.