re PR c/23228 (Silly "unused variable" warning after redeclaration of a local variable)

	PR c/23228
	* c-decl.c (pop_scope): Don't warn about an unused variable
	if it is marked with TREE_NO_WARNING.
	(duplicate_decls): Set TREE_NO_WARNING if olddecl and newdecl
	somenow mismatch and olddecl is to be replaced.

From-SVN: r105621