[ms-inline asm] Add support for parsing variables with namespace alias

[ms-inline asm] Add support for parsing variables with namespace alias
qualifiers.

This patch only adds support for parsing these identifiers in the
X86AsmParser.  The front-end interface isn't capable of looking up
these identifiers at this point in time.  The end result is the
compiler now errors during object file emission, rather than at
parse time.  Test case coming shortly.
Part of rdar://13499009 and PR13340

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178566 91177308-0d34-0410-b5e6-96231b3b80d8