Enable building directly with CMake

This makes building much faster (a no-op rebuild takes seconds instead
of minutes) and should make building more familiar for LLVM developers.

Author: mplatings

CHANGELOG.md

@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ### Added
 
 - A changelog
+- Support for building with CMake directly
 
 ### Changed