[clang] [CIR] Build out AST consumer patterns to reach the entry point into CIRGen (PR #91007)

[Sirui Mu via cfe-commits]

Lancern wrote:

FWIW, the current practice uses CamelCase for CIRGen and camelBack for all other CIR stuff. Most code in CIRGen is directly ported from clang CodeGen and the code style is kept as-is, while other part of CIR is invented from scratch and we follow MLIR style guides. I'm not sure whether this is an acceptable scheme, but the naming style problems pointed out specifically in this PR (e.g. `langOpts`) should be resolved if this scheme is to be followed as they appear in CIRGen.

https://github.com/llvm/llvm-project/pull/91007