[ms-inline asm] Have buildMSAsmString build a vector of unmodified AsmStrings.

Add a new static function, buildMSAsmPieces, that will break these strings down
into mnemonic and operands.  Upon a match failure, the idea is to use the
ErrorInfo from MatchInstructionImpl to inspect the mnemonic/operand and 
decide a course of action.  Unfortunately, there's no easy way to test this at
the moment.

llvm-svn: 162321