c++ modules: ICE with dynamic_cast [PR106304]

c++ modules: ICE with dynamic_cast [PR106304]

The FUNCTION_DECL we build for __dynamic_cast has an empty DECL_CONTEXT
but trees_out::tree_node expects FUNCTION_DECLs to have non-empty
DECL_CONTEXT, thus we crash when streaming out the dynamic_cast in the
below testcase.

This patch naively fixes this by setting DECL_CONTEXT for __dynamic_cast
appropriately.  I suppose we should push it into the namespace too, like
we do for __cxa_atexit which is similarly lazily declared.

	PR c++/106304

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_dynamic_cast_fn_p): Check for abi_node
	instead of global_namespace.
	* rtti.cc (build_dynamic_cast_1): Set DECL_CONTEXT and
	DECL_SOURCE_LOCATION when building dynamic_cast_node.  Push
	it into the namespace.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr106304_a.C: New test.
	* g++.dg/modules/pr106304_b.C: New test.