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.