[LibTooling] Add `before` and `after` selectors for selecting point-ranges relative to nodes.

Summary:
The `before` and `after` selectors allow users to specify a zero-length range --
a point -- at the relevant location in an AST-node's source.  Point ranges can
be useful, for example, to insert a change using an API that takes a range to be
modified (e.g. `tooling::change()`).

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62419

llvm-svn: 361955