[Fixed Point Arithmetic] Fixed Point Addition

[Fixed Point Arithmetic] Fixed Point Addition

This patch covers addition between fixed point types and other fixed point
types or integers, using the conversion rules described in 4.1.4 of N1169.

Usual arithmetic rules do not apply to binary operations when one of the
operands is a fixed point type, and the result of the operation must be
calculated with the full precision of the operands, so we should not perform
any casting to a common type.

This patch does not include constant expression evaluation for addition of
fixed point types. That will be addressed in another patch since I think this
one is already big enough.

Differential Revision: https://reviews.llvm.org/D53738

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@351364 91177308-0d34-0410-b5e6-96231b3b80d8