[CIR] clang-tidy CIRGenFunction.cpp (#1410)

Run clang-tidy on `CIRGenFunction.cpp` and fix all the issues that were
identified. The vast majority of issues were the case of parameter and
variable names. Some of the issues that didn't have to do with names had
to be resolved manually.

There should be no behavior changes.

Author: dkolsen-pgi