Now it's possible to allow sub-projects to define own checks and skip
"generic" ones.
To properly accomodate affected projects that might not have special
treatment we:
1. extend the set of affected projecs with dependent (e.g. add 'libc' if
'clang' was modified)
2. add custom steps for projects that define own workflow. At the moment
it's only libcxx and it has a custom trigger pipeline so it's noop.
3. add dependent projects and run generic check on them.
To illustrate: imagine that we have a dependency graph:
llvm -> clang -> openmp
and only clang was modified in a diff; also clang defines own checks.
Thus list of affected projects will be [clang, openmp].
After adding custom checks and removing their projecst: [openmp].
After adding dependencies: [llvm, clang, openmp]. Generic linux /
windows checks will be run on thouse 3 projects.
So as you can see in some scenarios projects with custom checks will
still go through generic checks.
Note that clang-format and clang-tidy checks are run only for "generic"
checks at the moment.
