[Sema] Add casting check for fixed to fixed point conversions

This change squelches the warning for a cast from fixed to fixed point
conversions when -Wbad-function-cast is enabled.

Fixes:

cast from function call of type '_Fract' to non-matching type '_Fract'
[-Wbad-function-cast]

Reviewed By: bjope

Differential Revision: https://reviews.llvm.org/D85157