* c-common.c (warn_abi): New variable.
	* c-common.h (warn_abi): Likewise.
	* c-opts.c (COMMAND_LINE_OPTIONS): Add -Wabi.
	(c_common_decode_option): Handle it.
	* doc/invoke.texi:P Document -Wabi.

	* class.c (layout_virtual_bases): Warn about bugs in G++ that
	result in incorrect object layouts.
	(layout_class_type): Likewise.

	* testsuite/g++.dg/abi/bitfield5.C: New test.
	* testsuite/g++.dg/abi/vbase10.C: Likewise.

From-SVN: r56618