Emit CK_NoOp casts in C mode, not just C++.

Previously, it had been using CK_BitCast even for casts that only
change const/restrict/volatile. Now it will use CK_Noop where
appropriate.

This is an alternate solution to r336746.

Differential Revision: https://reviews.llvm.org/D52918

llvm-svn: 343892