For example, we used to trigger CI even for commits that touched a file
whose path contained 'cmake', even if it's not the root cmake directory.
Fix that.
Now libc++ pipeline will be triggered from the "premerge-checks" and the
combined result are going to be returned to Harbormaster.
Reviewed-by: ldionne
Differential Revision: https://reviews.llvm.org/D89113
Louis Dionne