[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

llvm-svn: 179109