Use raw string literals to correctly stringify '"' and '\\' in attributes.
Only use an enum fwd declare if the enum is unknown.
Coding style.
Remove unused Decl2FwdDecl; build (unique) fwd decl Transaction.
Pass StringRef-s instead of copying strings...
Disable default cling-autoload callback (diag) for now.

The only real current user - ROOT - cannot make use of the diagnostics.
Instead the default callback should probably only be invoked if none of
the other, multiplexed handlers returns true.
Prefix annotation by $clingAutoload$ to not confuse it with others.
Use filename spelling as #included.
Simplify by using (hmm so sweet) raw string literals.
Use new fwd decl mechanism and raw string literals.
No need to be more const correct than getCI()->getSema()!
Adapt to new const-ness.
Missing newline.
Combine if-s; no functional change.
Do not double count for kTwoPasses.
Use fully qualified template args.
Collect dependencies of forward declarations.
Use cling's forward declaration features; ROOT part by Danilo and me.