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

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

Thanks @AaronBallman 

> But again, the important thing is to at least be self-consistent. e.g.,

It's a good point to reinforce, updated.

> I just don't think it benefits anyone to do so other than to make it easier to land the initial patches, which doesn't seem like a compelling reason to stick with auto.

Fair. I moved the usage of `auto` for `mlir::Location` for things outside "CIRGen/FrontendAction" but allowed as part of `lib/CIR/{Transforms,*}`, where the context makes it way more easier to understand (e.g. it can only be one thing, no Clang's source locations expected at that point)

> I think that's something we can live with, but like above, this makes it harder for the community in general while making it easier for folks already involved with MLIR.

Ok, I'll take the "we can live with" for this one.

Doc updated accordingly: https://llvm.github.io/clangir/GettingStarted/coding-guideline.html

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