[LibTooling] Update Stencil to use RangeSelector

Add support for creating a `StencilPart` from any `RangeSelector`, which
broadens the scope of `Stencil`.

Correspondingly, deprecate Stencil's specialized combinators `node` and `sNode`
in favor of using the new `selection` combinator directly (with the appropriate
range selector).

Reviewers: sbenza

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62160

llvm-svn: 361413