This adds APFixedPoint to the union of values that can be represented with an APValue.
Regarding tests, I couldn't actually find any under clang/unittests that test APValue specifically. Is it worth it to add some still?
One nit-pick, then this is good to go.
I know this is the established pattern, but could you just make MakeFixedPoint take a APFixedPoint && and move it into place instead of default-constructing one and then assigning over it?