[clang] [clang][NFC] Create CodeGenShared component for shared code with ClangIR (PR #157936)

[John McCall via cfe-commits]

rjmccall wrote:

Having library code that describes the layout of an RTTI object seems reasonable enough. I don't have strong opinions about whether that should go in AST or a new library.

I'm not sure I understand why ClangIR actually needs to lower RTTI objects, however. This seems like exactly the sort of thing that should not, in fact, be lowered in a high-level IR.

https://github.com/llvm/llvm-project/pull/157936