c++: #pragma GCC unroll C++ fixes [PR112795]

c++: #pragma GCC unroll C++ fixes [PR112795]

foo in the unroll-5.C testcase ICEs because cp_parser_pragma_unroll
during parsing calls maybe_constant_value unconditionally, which is
fine if !processing_template_decl, but can ICE otherwise.

While just calling fold_non_dependent_expr there instead could be enough
to fix the ICE (and I guess the right thing to do for backports if any),
I don't see a reason why we couldn't handle a dependent #pragma GCC unroll
argument as well, the unrolling isn't done in the FE and all the middle-end
cares about is that ANNOTATE_EXPR has a 1..65534 last operand when it is
annot_expr_unroll_kind.

So, the following patch changes all the unsigned short unroll arguments
to tree unroll (and thus avoids the tree -> unsigned short -> tree
conversions), does the type and value checking during parsing only if
the argument isn't dependent and repeats it during instantiation.

2023-12-04  Jakub Jelinek  <jakub@redhat.com>

	PR c++/112795
gcc/cp/
	* parser.cc (cp_parser_pragma_unroll): Use fold_non_dependent_expr
	instead of maybe_constant_value.
gcc/testsuite/
	* g++.dg/ext/unroll-5.C: New test.

(cherry picked from commit b6c78feea08c36e5754818c6a3d7536b3f8913dc)