Fix reverse to work on const rbegin()/rend().

Duncan found that reverse worked on mutable rbegin(), but the has_rbegin
trait didn't work with a const method.  See http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160815/382890.html
for more details.

Turns out this was already solved in clang with has_getDecl.  Copied that and made it work for rbegin.

This includes the tests Duncan attached to that thread, including the traits test.

llvm-svn: 278991