[Fixed Point Arithmetic] Fixed Point to Boolean Cast

[Fixed Point Arithmetic] Fixed Point to Boolean Cast

This patch is a part of https://reviews.llvm.org/D48456 in an attempt to split
the casting logic up into smaller patches. This contains the code for casting
from fixed point types to boolean types.

Differential Revision: https://reviews.llvm.org/D53308

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@345063 91177308-0d34-0410-b5e6-96231b3b80d8