[clang-tools-extra] [clang-tidy] Avoid processing declarations in system headers (PR #128150)

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

My proposal for this change is in `MatchASTConsumer` behind an off by default option and make clang tidy turn this on by default. Open a ticket about not considering the indirect field declarations top level. Properly update the documentation and the changelog. I think this approach has many technical and organizational advantages:
* Tidy continues to only use one ASTConsumer, so we incur less performance overhead. 
* Tools other than Tidy can adopt this option. This way the option can get more testing, and further improvements from authors of other tools. Tidy can benefit from these improvements.
* I anticipate that the code changes in `MatchASTConsumer` would not be more complicated than the code changes in this PR. We would still have small and incremental patches, complying to the LLVM dev policies. 
* This does not change the behavior of any other tools by default, so I do not anticipate push back from the community.

Given all of these, I would be interested to learn that what reasons do we have to go with this approach.

https://github.com/llvm/llvm-project/pull/128150