[ms-inline asm] Start tracking which tokens are registers and which are

[ms-inline asm] Start tracking which tokens are registers and which are
variables, function or label references.  The former is a potential clobber.
The latter is either an input or an output.  Unfortunately, it's difficult to
test this patch at the moment, but the added test case will eventually do so.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162026 91177308-0d34-0410-b5e6-96231b3b80d8