c: Support for -Wuseless-cast [PR84510]

Add support for Wuseless-cast C (and ObjC).

	PR c/84510

gcc/c/:
	* c-typeck.cc (build_c_cast): Add warning.

gcc/c-family/:
	* c.opt: Enable warning for C and ObjC.

gcc/:
	* doc/invoke.texi: Update.

gcc/testsuite/:
	* gcc.dg/Wuseless-cast.c: New test.