Allow RefactoringTool to write to memory instead of always to disk

RefactoringTool::run() always writes the result of rewrites to disk.
Instead, make this optional and provide a method for getting the
refactoring results in a memory buffer instead.

Also made ClangTool polymorphic so RefactoringTool could inherit from it
to properly express the IS-A relationship. This change also provides
access to ClangTool's public interface, e.g. mapVirtualFile() which is
important once refactored buffers start living in memory instead of on
disk.

Reviewers: klimek