swift234

[ASTPrinter] Simplify callbacks by dropping the notion of 'pending' callbacks

The three varieties of 'pending callbacks' made it hard to reason about
how callbacks would be presented to subclasses of ASTPrinter.  I
recently added new callbacks that would have made this even more complex
to deal with.

Luckily, it turns out they weren't buying us much, and simply forcing
any pending newlines and indentation to be printed ahead of making
callbacks was almost identical to the old behaviour. One complication is
that we now need to check for clang doc comments up front so we will
emit a newline in the right place.

This also incidentally fixed a bug in Loc vs Decl callback order.