re PR c++/54046 (wrong control reaches end of non-void function for switch case with throw and default)

	PR c++/54046
	* Makefile.in (gimple-low.o): Depend on langhooks.h.
	* gimple-low.c: Include langhooks.c.
	(block_may_fallthru): Handle TARGET_EXPR and ERROR_MARK,
	by default call lang_hooks.block_may_fallthru.
	* langhooks.h (struct lang_hooks): Add block_may_fallthru
	langhook.
	* langhooks-def.h (LANG_HOOKS_BLOCK_MAY_FALLTHRU): Define.
	(LANG_HOOKS_INITIALIZER): Use it.

	* cp-objcp-common.h (LANG_HOOKS_BLOCK_MAY_FALLTHRU): Redefine.
	* cp-objcp-common.c (cxx_block_may_fallthru): New function.
	* cp-tree.h (cxx_block_may_fallthru): New prototype.

	* g++.dg/warn/Wreturn-type-8.C: New test.

From-SVN: r193762