[Clang] Improve CodeGenerator API a bit (#175239)

Essentially, figuring out how to use `CodeGenerator` was very confusing
to me and I figured the API could be improved a bit, so:
- the `CodeGenerator` ctor is now protected since an instance of
`CodeGenerator` that is not a `CodeGeneratorImpl` is a bit useless (and
deriving from it and implementing it yourself honestly just defeats the
point of using this to begin with);
- `ReleaseModule()` releases ownership of the module, so it should
return a `unique_ptr`;
- `CreateLLVMCodeGen()` also returns a `unique_ptr` now;
- added a `CreateLLVMCodeGen()` overload that takes a
`CompilerInstance&` and uses some of its state instead of requiring the
user to pass everything in manually; this is consistent w/ other parts
of our API, and most uses of this function in the codebase can be
refactored to use that overload instead (and a code search I did also
showed that a lot of people that use this API also just use the state
from a `CompilerInstance`).

I should have liked to replace `CreateLLVMCodeGen` w/
`CodeGenerator::Create`, but there are a lot of uses of
`CreateLLVMCodeGen()` in the wild, so the only thing we could do is keep
`CreateLLVMCodeGen()` and deprecate it, and at that point I don’t think
it’s really worth it; I added a comment to the `CodeGenerator()`
constructor declaration that points to it though.

Fixes #172169.