[ms-inline asm] Add a few new APIs to the AsmParser class in support of MS-Style

inline assembly.  For the time being, these will be called directly by clang.
However, in the near future I expect these to be sunk back into the MC layer
and more basic APIs (e.g., getClobbers(), getConstraints(), etc.) will be called
by clang.

llvm-svn: 165946