Implement fixed-point arithmetic

[dkolsen-pgi]

The ClangIR code is littered with llvm_unreachable or assertions about fixed-point not being implemented yet. Almost all code that requires the -ffixed-point option to get through the Clang front end will hit one of these not-yet-implemented errors. Fixed-point supports needs to be implemented at some point.

This is a big enough task that it should probably be broken into smaller tasks, but I don't know what the correct granularity is, so I am not creating those subtasks yet.