[clang-tidy] Add a flag to enable alpha checkers

Summary: The alpha checkers can already be enabled using the clang driver, this allows them to be enabled using the clang-tidy as well. This can make it easier to test the alpha checkers with projects which already support the compile_commands.json. It will also allow more people to give feedback and patches about the alpha checkers since they can run it as part of clang tidy checks.

Reviewers: aaron.ballman, hokein, ilya-biryukov, alexfh, lebedev.ri, xbolva00

Reviewed By: aaron.ballman, alexfh, lebedev.ri, xbolva00

Subscribers: xbolva00, NoQ, dcoughlin, lebedev.ri, xazax.hun, cfe-commits

Patch by Paul Fultz II!

Differential Revision: https://reviews.llvm.org/D46159

llvm-svn: 332609