typechecker: recognize binary int literals

Summary: The binary int syntax (##0b101## for ##5##) is a handy syntax
 for certain use cases (such as declaring constants for bitwise
 masks), which wasn't being lexed as an int literal as an oversight.
 As with ##0x##, we don't recognize the alternate form ##0B##.

Reviewed By: @jwatzman

Differential Revision: D1533071