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

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

I encoded the discussion in our clangir.org page, with some speculation on Aaron's pending replies, happy to change and fix anything. In the future we'll upstream that too, but for the time being it's here: https://llvm.github.io/clangir/GettingStarted/coding-guideline.html

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