[libTooling] In Transformer, generalize `applyFirst` to admit rules with incompatible matchers.

Summary:
This patch removes an (artificial) limitation of `applyFirst`, which requires
that all of the rules' matchers can be grouped together in a single `anyOf()`.
This change generalizes the code to group the matchers into separate `anyOf`s
based on compatibility. Correspondingly, `buildMatcher` is changed to
`buildMatchers`, to allow for returning a set of matchers rather than just one.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65877

llvm-svn: 368681