c++: Further #pragma GCC unroll C++ fix [PR112795]

When committing the #pragma GCC unroll patch, I found I forgot one spot
for diagnosting the invalid unrolls - if #pragma GCC unroll argument is
dependent and the pragma is before a range for loop, the unroll tree (now,
before one converted form ushort) is saved into RANGE_FOR_UNROLL and
tsubst_stmt was RECURing on it, but didn't diagnose if it was invalid and
so we ICEd later in the middle-end when  ANNOTATE_EXPR had unexpected
argument.

The following patch fixes that.  So that the diagnostics isn't done in 3
different places, the patch introduces a new function that both
cp_parser_pragma_unroll and instantiation of ANNOTATE_EXPR and RANGE_FOR_STMT
can use.

2023-12-05  Jakub Jelinek  <jakub@redhat.com>

	PR c++/112795
	* cp-tree.h (cp_check_pragma_unroll): Declare.
	* semantics.cc (cp_check_pragma_unroll): New function.
	* parser.cc (cp_parser_pragma_unroll): Use cp_check_pragma_unroll.
	* pt.cc (tsubst_expr) <case ANNOTATE_EXPR>: Likewise.
	(tsubst_stmt) <case RANGE_FOR_STMT>: Likwsie.

	* g++.dg/ext/unroll-2.C: Use { target c++11 } instead of dg-skip-if for
	-std=gnu++98.
	* g++.dg/ext/unroll-3.C: Likewise.
	* g++.dg/ext/unroll-7.C: New test.
	* g++.dg/ext/unroll-8.C: New test.