Add a flag that controls if clang-tidy and clang-include-fixer are built into libclang.

Based on the discussion on D55415, also make the flag default to false.
Having libclang depend on clang-tools-extra means check-clang builds all
of clang-tools-extra, which besides being a layering violation takes
quite some time, since clang-tools-extra has many files that are slow
to compile.

Longer term, we likely will want to remove this flag completely. If
people need this functionality, maybe there could be a
libclang-tools-extra that's libclang + clang-tidy and
clang-includes-fixer linked in.

Differential Revision: https://reviews.llvm.org/D79599