[ms-inline asm] Add support for lexing binary integers with a [bB] suffix.

This is complicated by backward labels (e.g., 0b can be both a backward label
and a binary zero).  The current implementation assumes [0-9]b is always a
label and thus it's possible for 0b and 1b to not be interpreted correctly for
ms-style inline assembly.  However, this is relatively simple to fix in the
inline assembly (i.e., drop the [bB]).

This patch also limits backward labels to [0-9]b, so that only 0b and 1b are
ambiguous.
Part of rdar://12470373