[ms-inline asm] Address the FIXME in AsmParser.cpp.

// FIXME: Constraints are hard coded to 'm', but we need an 'r'
// constraint for addressof.  This needs to be cleaned up!

Test cases are already in place.  Specifically,
clang/test/CodeGen/ms-inline-asm.c t15(), t16(), and t24().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172569 91177308-0d34-0410-b5e6-96231b3b80d8