[refactor] Add the AST source selection component

This commit adds the base AST source selection component to the refactoring
library. AST selection is represented using a tree of SelectedASTNode values.
Each selected node gets its own selection kind, which can actually be None even
in the middle of tree (e.g. statement in a macro whose child is in a macro
argument). The initial version constructs a "raw" selection tree, without
applying filters and canonicalisation operations to the nodes.

Differential Revision: https://reviews.llvm.org/D35012

llvm-svn: 311655