c++: ICE with -Wall and constexpr if [PR94937]

c++: ICE with -Wall and constexpr if [PR94937]

An ICE arises here because we call cp_get_callee_fndecl_nofold in a
template, and we've got a CALL_EXPR whose CALL_EXPR_FN is a BASELINK.
This tickles the INDIRECT_TYPE_P assert in cp_get_fndecl_from_callee.

Fixed by turning the assert into a condition and returning NULL_TREE
in that case.

	PR c++/94937
	* cvt.c (cp_get_fndecl_from_callee): Return NULL_TREE if the function
	type is not INDIRECT_TYPE_P.
	* decl.c (omp_declare_variant_finalize_one): Call
	cp_get_callee_fndecl_nofold instead of looking for the function decl
	manually.

	* g++.dg/cpp1z/constexpr-if34.C: New test.
	* g++.dg/cpp2a/is-constant-evaluated10.C: New test.