Enable release build for OCLint code

In order to leverage the optimized version of LLVM/Clang codebase, we now are able to build OCLint in release mode as well. This will largely improve the performance in the runtime, accelerate the analysis speed significantly, and reduce the file size dramatically.

It's highly recommended to build OCLint in release mode for production environment, like development machine and continuous integration server. Please be aware that, `-debug` option won't work in release mode, so build the project in debug mode when users have obvious intent to debug the intermediate progress of the program invocation.