C and C++ FE changes to support flexible array members in unions and alone in structu...

C and C++ FE changes to support flexible array members in unions and alone in structures. Adjust testcases for flexible array member in union and alone in structure extension.

PR c/53548

gcc/c/ChangeLog:

	PR c/53548
	* c-decl.cc (finish_struct): Change errors to pedwarns for the cases
	flexible array members in union or alone in structures.

gcc/cp/ChangeLog:

	PR c/53548
	* class.cc (diagnose_flexarrays): Change error to pdewarn for the case
	flexible array members alone in structures.
	* decl.cc (grokdeclarator): Change error to pdewarn for the case
	flexible array members in unions.

gcc/ChangeLog:

	PR c/53548
	* stor-layout.cc (place_union_field): Use zero sizes for flexible array
	member fields.

gcc/testsuite/ChangeLog:

	PR c/53548
	* c-c++-common/builtin-clear-padding-3.c: Adjust testcase.
	* g++.dg/ext/flexary12.C: Likewise.
	* g++.dg/ext/flexary19.C: Likewise.
	* g++.dg/ext/flexary2.C: Likewise.
	* g++.dg/ext/flexary3.C: Likewise.
	* g++.dg/ext/flexary36.C: Likewise.
	* g++.dg/ext/flexary4.C: Likewise.
	* g++.dg/ext/flexary5.C: Likewise.
	* g++.dg/ext/flexary8.C: Likewise.
	* g++.dg/torture/pr64280.C: Likewise.
	* gcc.dg/20050620-1.c: Likewise.
	* gcc.dg/940510-1.c: Likewise.