[LibTooling] Update Transformer to use RangeSelector instead of NodePart enum.

Transformer provides an enum to indicate the range of source text to be edited.
That support is now redundant with the new (and more general) RangeSelector
library, so we remove the custom enum support in favor of supporting any
RangeSelector.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62149

llvm-svn: 361392