[libTooling] Generalize string explanation as templated metadata

Change RewriteRule from holding an `Explanation` to being able to generate
arbitrary metadata. Where TransformerClangTidyCheck was interested in a string
description for the diagnostic, other tools may be interested in richer metadata
at a higher level of abstraction than at the edit level (which is currently
available as ASTEdit::Metadata).

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D120360