Configure default value of LLVM_ENABLE_WARNINGS in HandleLLVMOptions.cmake.
LLVM_ENABLE_WARNINGS is documented as ON by default, but HandleLLVMOptions assumes the default has been set somewhere else. If it has not been explicitly set, then HandleLLVMOptions implicitly uses OFF as a default.
This removes the various option() declarations in favor of a single declaration in HandleLLVMOptions. This will prevent the unwanted use of -w that is mentioned in a couple of the comments.