MS ABI: Add support for mangling __restrict

Pointer types in the MSVC ABI are a bit awkward, the width of the
pointer is considered a kind of CVR qualifier.

Restrict is handled similarly to const and volatile but is mangled after
the pointer width qualifier.

This fixes PR18880.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201569 91177308-0d34-0410-b5e6-96231b3b80d8