[analyzer] Don't display implementation checkers under -analyzer-checker-help, but...

[analyzer] Don't display implementation checkers under -analyzer-checker-help, but do under the new flag -analyzer-checker-help-hidden

During my work on analyzer dependencies, I created a great amount of new
checkers that emitted no diagnostics at all, and were purely modeling some
function or another.

However, the user shouldn't really disable/enable these by hand, hence this
patch, which hides these by default. I intentionally chose not to hide alpha
checkers, because they have a scary enough name, in my opinion, to cause no
surprise when they emit false positives or cause crashes.

The patch introduces the Hidden bit into the TableGen files (you may remember
it before I removed it in D53995), and checkers that are either marked as
hidden, or are in a package that is marked hidden won't be displayed under
-analyzer-checker-help. -analyzer-checker-help-hidden, a new flag meant for
developers only, displays the full list.

Differential Revision: https://reviews.llvm.org/D60925

llvm-svn: 359720