[clang] [clang][CIR] Move CIRGen types into clang::CIRGen (PR #115385)

[Shoaib Meenai via cfe-commits]

smeenai wrote:

Stack from [ghstack](https://github.com/ezyang/ghstack) (oldest at bottom):
* (to be filled)

https://github.com/llvm/llvm-project/pull/115385