Print a single line listing all the categories that are being skipped,
rather than relying on the check.*Support() functions specifying why a
particular category will be skipped. If we know why a category got
skipped, still print that in verbose mode.
The motivation for this change is that sometimes engineers misidentify
the output of these messages as the cause for a test failure (e.g. not
being able to build libc++ or libstdc++).
Differential revision: https://reviews.llvm.org/D100508
6ce1067f2d