Suppress MSVC's equivalent of -Wshadow warnings

IMO we need to clean up some of these, but the member variable one
(C4458) has false positives on static methods.  It is currently firing
on Twine, which has a static method like:
  struct Twine {
    uintptr_t LHS, RHS;
    static void staticMethod() {
      // warning C4458: declaration of 'LHS' hides class member
      uintptr_t LHS;
      ...
    }
  };

We should fix up clang's -Wshadow and clean it up, and then we can
re-enable some of these MSVC warnings.

llvm-svn: 221012