Remove vtables from the Stmt hierarchy; this was pretty easy as

there were only three virtual methods of any significance.

The primary way to grab child iterators now is with
  Stmt::child_range children();
  Stmt::const_child_range children() const;
where a child_range is just a std::pair of iterators suitable for
being llvm::tie'd to some locals.  I've left the old child_begin()
and child_end() accessors in place, but it's probably a substantial
penalty to grab the iterators individually now, since the
switch-based dispatch is kindof inherently slower than vtable
dispatch.  Grabbing them together is probably a slight win over the
status quo, although of course we could've achieved that with vtables, too.

I also reclassified SwitchCase (correctly) as an abstract Stmt
class, which (as the first such class that wasn't an Expr subclass)
required some fiddling in a few places.

There are somewhat gross metaprogramming hooks in place to ensure
that new statements/expressions continue to implement
getSourceRange() and children().  I had to work around a recent clang
bug;  dgregor actually fixed it already, but I didn't want to
introduce a selfhosting dependency on ToT.

llvm-svn: 125183