In MC asm parsing, account for the possibility of whitespace within

In MC asm parsing, account for the possibility of whitespace within
the "identifier" parsed by the frontend callback by skipping forward
until we've consumed a token that ends at the point dictated by the
callback.

In addition, inform the callback when it's parsing an unevaluated
operand (e.g. mov eax, LENGTH A::x) as opposed to an evaluated one
(e.g. mov eax, [A::x]).

This commit depends on a clang commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180978 91177308-0d34-0410-b5e6-96231b3b80d8