Heads up: CMAKE_BUILD_TYPE is now a required variable to build CMake

Just wanted to let everyone know that ⚙ D124153 [CMake] Make omitting CMAKE_BUILD_TYPE an error landed and that now means that if you run CMake on LLVM without specifying CMAKE_BUILD_TYPE it will result in an error.

Documentation about CMAKE_BUILD_TYPE has also been improved to help new users: Building LLVM with CMake — LLVM 15.0.0git documentation