matchers_sample almost ready
matchers_sample almost ready

- cleaned up the construction of SourceManager to be shorter
- added some comments

Need a bit more comments. Then tests and it can move out of experimental/

eliben/llvm-clang-samples,eliben/llvm-clang-samples,eliben/llvm-clang-samples,eliben/llvm-clang-samples,eliben/llvm-clang-samples,eliben/llvm-clang-samples