Update the C FE routine "add_flexible_array_elts_to_size" C++ FE routine "layout_var_...

Update the C FE routine "add_flexible_array_elts_to_size" C++ FE routine "layout_var_decl" to handle the cases when the DECL is union.

PR c/53548

Add testing cases to test the _bos for flexible array members in unions
or alone in structures.

gcc/c/ChangeLog:

	PR c/53548
	* c-decl.cc (add_flexible_array_elts_to_size): Handle the cases
	when the DECL is union.

gcc/cp/ChangeLog:

	PR c/53548
	* decl.cc (layout_var_decl): Handle the cases when the DECL is
	union with a flexible array member initializer.

gcc/testsuite/ChangeLog:

	PR c/53548
	* c-c++-common/fam-in-union-alone-in-struct-bos-1.c: New test.
	* c-c++-common/fam-in-union-alone-in-struct-bos.c: New test.