[Fixed Point Arithmetic] FixedPointCast

This patch is a part of https://reviews.llvm.org/D48456 in an attempt to
split them up. This contains the code for casting between fixed point types
and other fixed point types.

The method for converting between fixed point types is based off the convert()
method in APFixedPoint.

Differential Revision: https://reviews.llvm.org/D50616

llvm-svn: 344530