The former is correct - you only need to build LLVM and Clang in debug mode if you want to debug the compiler. If you want to debug the application produced, you need to compile it with debug symbols - i.e. pass the -g
flag to Clang when you build your program - and that is enabled no matter in what mode LLVM and Clang were built.
So for your needs, you should compile in release mode.
As for your question - why is that the default - I'm guessing it's because it is assumed that if you wanted to just run it, you'd just obtain a pre-built binary instead of downloading and building the source code.
Also, regarding that last question you sneaked in - I believe it is referring to tools from this list of LLVM tools.