Add a method to get the list of registered static analyzer checkers.

Summary:
This provides a better interface for clang-tidy and encapsulates the knowledge
about experimental checkers instead of leaving this to the clients.

Reviewers: zaks.anna

Subscribers: a.sidorin, NoQ, dcoughlin, cfe-commits

Differential Revision: https://reviews.llvm.org/D26310

llvm-svn: 286218