[ms-inline asm] Extend the MC AsmParser API to match MCInsts (but not emit).

This new API will be used by clang to parse ms-style inline asms.

One goal of this project is to use this style of inline asm for targets other
then x86.  Therefore, this API needs to be implemented for non-x86 targets at
some point in the future.

llvm-svn: 161624