[PATCH] D98710: [clang-tidy] New feature --skip-headers, part 1, setTraversalScope

[Chih-Hung Hsieh via Phabricator via cfe-commits]

chh added a comment.

Some Android developers and legacy code care less about clang-tidy warnings.
Newer developers spend a lot of time to get lint-free new code.
So Android source tree has a lot of clang-tidy flags like
header-filter, checks, and warnings-as-errors to select checks
for different modules. People are even asking for new features like
making header-filter to accept a list of regexp like the checks flag.

If a new change to header-filter or skip-headers lose existing valid warnings,
maybe it will be perfectly acceptable to some but not to people who want to
make code lint-free. To them, these changes are regressions.
A clang-tidy check's owner would not like to lose capability for
some minor performance gain, either.

bugprone-forward-declaration-namespace is just one valid warning found
in a couple of days. Android tree is huge and has disabled many checks,
so we won't know the impact to all checks any time soon. 
If we have a mechanism to keep any special checks from the impact,
like the PPCallback-based checks, it will be much safer to release 
the first phase skip-headers and know that we can fix any bad impact quickly.

I am trying such a mechanism to support "see-all-file" MatchFinder-based checks.
Those checks will be very few, so skip-headers still saves a lot of runtime.
After more tests, I will upload it to D98709 <https://reviews.llvm.org/D98709>.

Please note that I added a new test skip-headers-2.cpp.
It is extremely simplified, but reflects the need to check a top-level
Decl and ignore warnings in included nested Decls in other files.
If we implement skip-headers based on only top-level Decls,
we won't be able to support such a use case.
In D98709 <https://reviews.llvm.org/D98709>, we check/skip not only top-level Decls.


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https://reviews.llvm.org/D98710