c: Fix bug in typedef redefinitions of tagged types [PR118765]

c: Fix bug in typedef redefinitions of tagged types [PR118765]

When we redefine a tagged type we incorrectly update TYPE_STUB_DECL
of the previously defined type instead of the new one.  Because
TYPE_STUB_DECL is used when determining whether two such types are
the same, this can cause valid typedef redefinitions to be rejected
later. This is only a partial fix for PR118765.

	PR c/118765

gcc/c/ChangeLog:
	* c-decl.cc (finish_struct,finish_enum): Swap direction when
	copying TYPE_STRUB_DECL in redefinitions.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr118765.c: New test.