re PR c/13656 (ICE after redeclaring size_t in the system headers)

re PR c/13656 (ICE after redeclaring size_t in the system headers)

	PR 13656
	* c-decl.c (diagnose_mismatched_decls): Whenever newtype or
	oldtype is set, set *newtypep or *oldtypep too.  Do not set
	them at the very end.
	(validate_proto_after_old_defn): Restructure for comprehensibility;
	make error messages clearer.
testsuite:
	* gcc.dg/typedef-redecl.c: New test case.
	* gcc.dg/typedef-redecl.h: New support file.

From-SVN: r75787