[clang-tidy] Make ClangTidyCheck::OptionsView public.

Summary: The `OptionsView` class is currently protected. This constraint prevents tidies from passing the OptionsView to, for example, a helper function. Similarly, TransformerClangTidyCheck cannot pass the `OptionsView` object to functions that generate `tooling::RewriteRule`s.  The latter is needed to allow the definition of such rules to depend on the clang-tidy options, as demonstrated in the child revision.

Reviewers: gribozavr

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63287

llvm-svn: 363296