Improves RecursiveASTVisitor:

* refactors code in DEF_TRAVERSE_DECL() into
  TraverseDeclContextHelper() to reduce code bloat and facilitate
  reuse.
* makes the order of traversing a FunctionDecl (including its
  subclasses)'s children more natural: before the function parameters
  are visited after the function body; now they are visited after the
  function type and before the function body.
* fixes a double count for function return type and arguments.

Reviewed by chandlerc and csilvers.

llvm-svn: 106236