[clang-tidy] Untangle layering in ClangTidyDiagnosticConsumer somewhat. NFC

Summary:
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.

Reviewers: hokein

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D54033

llvm-svn: 346418