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

[Mehdi Amini via cfe-commits]

joker-eph wrote:

What part of the "camelBack" is visible to the user of the API? 
(and in general the method prototype in the header should likely use the same parameter names as the implementation file)

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