PR c++/99251 - inconsistent -Wnonnull warning behaviour with dynamic_cast

PR c++/99251 - inconsistent -Wnonnull warning behaviour with dynamic_cast

gcc/cp/ChangeLog:

	PR c++/99251
	* class.c (build_base_path): Call build_if_nonnull.
	* cp-tree.h (build_if_nonnull): Declare.
	* rtti.c (ifnonnull): Rename...
	(build_if_nonnull): ...to this.  Set no-warning bit on COND_EXPR.
	(build_dynamic_cast_1): Adjust to name change.

gcc/testsuite/ChangeLog:

	PR c++/99251
	* g++.dg/warn/Wnonnull9.C: Expect no warnings.
	* g++.dg/warn/Wnonnull12.C: New test.