Several changes to CMakeLists.txt and FindLLVM.cmake

- Fix typo in comment
- Removes traces from LLVM 3.0 support
- Fix the regular expression used to match LLVM version
- Add support for LLVM 3.6 config tool
Fix deprecation warning
Fix bug in llvm.ctpop intrinsic.

Just use the simple algorithm from dmd 2.067.