[Syntax] Add iterators over children of syntax trees.

This gives us slightly nicer syntax (foreach) for idioms currently expressed
as a loop, and the option to use range algorithms where it makes sense
(e.g. llvm::all_of et al encapsulate the needed flow control in a useful way).

It's also a building block for iteration over filtered views (e.g. iterate over
all Stmt children, with the right type):
for (const Statement &S : filter<Statement>(N.children()))
  ...

I realize the recent direction has been mostly towards strongly-typed
node-specific facilities, but I think it's important we have convenient
generic facilities too.

Differential Revision: https://reviews.llvm.org/D90023