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/
	* cp-tree.h (cp_convert_range_for): Change UNROLL type from
	unsigned short to tree.
	(finish_while_stmt_cond, finish_do_stmt, finish_for_cond): Likewise.
	* parser.cc (cp_parser_statement): Pass NULL_TREE rather than 0 to
	cp_parser_iteration_statement UNROLL argument.
	(cp_parser_for, cp_parser_c_for): Change UNROLL type from
	unsigned short to tree.
	(cp_parser_range_for): Likewise.  Set RANGE_FOR_UNROLL to just UNROLL
	rather than build_int_cst from it.
	(cp_convert_range_for, cp_parser_iteration_statement): Change UNROLL
	type from unsigned short to tree.
	(cp_parser_omp_loop_nest): Pass NULL_TREE rather than 0 to
	cp_parser_range_for UNROLL argument.
	(cp_parser_pragma_unroll): Return tree rather than unsigned short.
	If parsed expression is type dependent, just return it, don't diagnose
	issues with value if it is value dependent.
	(cp_parser_pragma): Change UNROLL type from unsigned short to tree.
	* semantics.cc (finish_while_stmt_cond): Change UNROLL type from
	unsigned short to tree.  Build ANNOTATE_EXPR with UNROLL as its last
	operand rather than build_int_cst from it.
	(finish_do_stmt, finish_for_cond): Likewise.
	* pt.cc (tsubst_stmt) <case RANGE_FOR_STMT>: Change UNROLL type from
	unsigned short to tree and set it to RECUR on RANGE_FOR_UNROLL (t).
	(tsubst_expr) <case ANNOTATE_EXPR>: For annot_expr_unroll_kind repeat
	checks on UNROLL value from cp_parser_pragma_unroll.
gcc/testsuite/
	* g++.dg/ext/unroll-5.C: New test.
	* g++.dg/ext/unroll-6.C: New test.