[AST] Make RecursiveASTVisitor visit TemplateDecls in source order

Summary:
This causes template arguments to be traversed before the templated
declaration, which is useful for clients that expect the nodes in
the same order as they are in the source code. Additionally, there
seems to be no good reason not to do so.

This was moved here from LexicallyOrderedRecursiveASTVisitor. The tests
still reside in LexicallyOrderedRecursiveASTVisitorTest.cpp under
VisitTemplateDecls.

Reviewers: arphaman, rsmith, klimek

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D37662

llvm-svn: 312911