[LibCalls] Cast Char argument to 'int' before calling emitFPutC

The helpers in BuildLibCalls normally expect that the Value
arguments already have the correct type (matching the lib call
signature). And exception has been emitFPutC which casted the Char
argument to 'int' using CreateIntCast. This patch moves the cast to
the caller instead of doing it inside emitFPutC.

I think it makes sense to make the BuildLibCall API:s a bit
more consistent this way, despite the need to handle the int cast
in two different places now.

Differential Revision: https://reviews.llvm.org/D135066