[clang-tidy] Untangle layering in ClangTidyDiagnosticConsumer somewhat. NFC
Clang's hierarchy is CompilerInstance -> DiagnosticsEngine -> DiagnosticConsumer.
(Ownership is optional/shared, but this structure is fairly clear).
Currently ClangTidyDiagnosticConsumer *owns* the DiagnosticsEngine:
- this inverts the hierarchy, which is confusing
- this means ClangTidyDiagnosticConsumer() mutates the passed-in context, which is both surprising and limits flexibility
- it's not possible to use a different DiagnosticsEngine with ClangTidy
This means a little bit more code in the places ClangTidy is used standalone,
but more flexibility in using ClangTidy with other diagnostics configurations.
Subscribers: xazax.hun, cfe-commits
Differential Revision: https://reviews.llvm.org/D54033