Fixing MSVC warning: "warning C4373: 'blah': virtual function overrides 'blah', previous versions of the compiler did not override when parameters only differed by const/volatile qualifiers."

This warning is basically useless because the "previous versions" being referred to is MSVC 2005 and earlier, and we obviously do not care about them. However, this warning isn't particularly chatty (I don't recall ever seeing it previously), and it has the opportunity to point out cases where the cv-qualifiers differ unintentionally (like this case), so I am leaving it enabled for now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216267 91177308-0d34-0410-b5e6-96231b3b80d8