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

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

Building a full parent map makes sense to me, but I have mixed feelings about modifying the `ParentMap` ctor that everyone uses. What if one of the clients do not want a full map? I think the change might need to be more targeted. Either passing on option to `ParentMap` or create a new overload for the ctor and update the use in the matcher library. 

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