[CodeGen] Use the common semantic for fixed-point codegen, not the result semantic.

Summary:
Using the result semantic is wrong in some cases, such as
unsigned fixed-point + signed integer. In this case, the
result semantic is unsigned and the common semantic is
signed.

Reviewers: leonardchan

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82662