[clang-tidy] In TransformerClangTidyCheck, require Explanation field.

Summary:
In general, the `Explanation` field is optional in `RewriteRule` cases. But,
because the primary purpose of clang-tidy checks is to provide users with
diagnostics, we assume that a missing explanation is a bug.  This change adds an
assertion that checks all cases for an explanation, and updates the code to rely
on that assertion correspondingly.

Reviewers: ilya-biryukov

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62340

llvm-svn: 361647