Fix conversion from bool to floating point value

Fix conversion from bool to floating point value

Summary:
Due to the definition of how floating point values convert to boolean,
the check for undefined behaviour wouldn't work correctly. The result
of (1 - 0.9999999999999999) would yield 0 when converted to an integer,
but yields true (1) when converted to a boolean.

As all floating point values can thus be converted to boolean without
triggering undefined behaviour, this change overloads checkConversion()
appropriately

Reviewed By: yfeldblum

Differential Revision: D3477368

fbshipit-source-id: 5b2aeb6194629cf3a6195529aac2362c0d35799c