MS asm: Properly handle quoted symbol names

We would get confused by '@' characters in symbol names, we would
mistake the text following them for the variant kind.

When an identifier a string, the variant kind will never show up inside
of it.  Instead, check to see if there is a variant following the
string.

This fixes PR19965.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211249 91177308-0d34-0410-b5e6-96231b3b80d8