Get FunctionDecl to recurse on FunctionTypeLoc rather than

FunctionType!  I didn't realize it was available, until rjmccall
pointed out that DeclaratorDecl made the typeloc available.  This
makes FunctionDecl recursion *much* easier, because the typeloc can
take care of default parameters, so we no longer have to do that
separately, which means we can just do a normal type traversal instead
of this special-case WalkUp stuff we did before.

The only downside -- and it's minor -- is that because the TypeLoc
handles both the return type and the argument types, we can't recurse
on the explicit template args in the right place (which would be
between them).  I do it beforehand instead.  So for
   int MyFunc<float>(char x);
we get callbacks in the order: float, int, char.

Reviewed by chandlerc

llvm-svn: 116945