c: fix ICE when forming composite type for two structures / unions [PR117548]

When forming the composite type from two tagged type, we need to find the
original type for a typedecl to get the correct tag.

	PR c/117548

gcc/c/ChangeLog:
	* c-decl.cc (finish_struct): Add checking assertion.
	* c-typeck.cc (c_type_original): New function.
	(composite_types_internal): Get tag from original type.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr117548.c: New test.