c++: -Wuninitialized for mem-inits and empty classes [PR19808]

c++: -Wuninitialized for mem-inits and empty classes [PR19808]

This fixes a bogus -Wuninitialized warning: there's nothing to initialize
in empty classes, so don't add them into our uninitialized set.

	PR c++/19808

gcc/cp/ChangeLog:

	* init.c (emit_mem_initializers): Don't add is_really_empty_class
	members into uninitialized.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wuninitialized-28.C: Make a class nonempty.
	* g++.dg/warn/Wuninitialized-29.C: Likewise.
	* g++.dg/warn/Wuninitialized-31.C: New test.