ms-inline-asm: Fix parsing label names inside bracket expressions

Summary:
This fixes a couple of issues.  One is ensuring that AOK_Label rewrite
rules have a lower priority than AOK_Skip rules, as AOK_Skip needs to
be able to skip the brackets properly.  The other part of the fix ensures
that we don't overwrite Identifier when looking up the identifier, and
that we use the locally available information to generate the AOK_Label
rewrite in ParseIntelIdentifier.  Doing that in CreateMemForInlineAsm
would be problematic since the Start location there may point to the
beginning of a bracket expression, and not necessarily the beginning of
an identifier.

This also means that we don't need to carry around the InternlName field,
which helps simplify the code.

Test Plan: This will be tested on the clang side.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5445