c: Fix up __builtin_assoc_barrier handling in the C FE [PR104427]

c: Fix up __builtin_assoc_barrier handling in the C FE [PR104427]

The following testcase ICEs, because when creating PAREN_EXPR for
__builtin_assoc_barrier the FE doesn't do the usual tweaks for
EXCESS_PRECISION_EXPR or C_MAYBE_CONST_EXPR.  I believe that the
declared effect of the builtin is just association barrier, so
e.g. excess precision should be still handled like if it wasn't
there.

The following patch uses build_unary_op to handle those.

2022-02-09  Jakub Jelinek  <jakub@redhat.com>

	PR c/104427
	* c-parser.cc (c_parser_postfix_expression)
	<case RID_BUILTIN_ASSOC_BARRIER>: Use parser_build_unary_op
	instead of build1_loc to build PAREN_EXPR.
	* c-typeck.cc (build_unary_op): Handle PAREN_EXPR.
	* c-fold.cc (c_fully_fold_internal): Likewise.

	* gcc.dg/pr104427.c: New test.