Fix parsing of binary literals denoted with a capital '0B'

Summary: The binary prefix should be case-insensitive,
but the scalar expression parser was only checking lower-case.

Closes #4253

Reviewed By: @nigelchanyk

Differential Revision: D1684154

Signature: t1:1684154:1416146734:6f4954beaa2e96c78567b37c658f72d59894c73b