MS asm: Attempt to parse variables followed by a bracketed displacement

This is required to include MSVC's <atomic> header, which we do now in
LLVM.

Tests forthcoming in Clang, since that's where we test semantic inline
asm changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202865 91177308-0d34-0410-b5e6-96231b3b80d8