Move parsing of identifiers in MS-style inline assembly into
the actual parser and support arbitrary id-expressions.

We're actually basically set up to do arbitrary expressions here
if we wanted to.

Assembly operands permit things like A::x to be written regardless
of language mode, which forces us to embellish the evaluation
context logic somewhat.  The logic here under template instantiation
is incorrect;  we need to preserve the fact that an expression was
unevaluated.  Of course, template instantiation in general is fishy
here because we have no way of delaying semantic analysis in the
MC parser.  It's all just fishy.

I've also fixed the serialization of MS asm statements.

This commit depends on an LLVM commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180976 91177308-0d34-0410-b5e6-96231b3b80d8