[ms-inline asm] Maintain a StringRef to reference a symbol in a parsed operand,
rather than deriving the StringRef from the Start and End SMLocs.

Using the Start and End SMLocs works fine for operands such as [Symbol], but
not for operands such as [Symbol + ImmDisp].  All existing test cases that
reference a variable exercise this patch.
rdar://13602265



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179109 91177308-0d34-0410-b5e6-96231b3b80d8