c: Fix recursive structure / union redeclaration with qualifiers [PR124303]

We reject correct recursive redeclarations when qualifiers are involved.
The reason is that the check is done before the variants are completed.

	PR c/124303

gcc/c/ChangeLog:
	* c-decl.cc (finish_struct): Check for consistency of
	declarations after completing variants.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr124303.c: New test.

(cherry picked from commit 7c3e6df8ae01a473fba63e12efe2c82bd50ad502)