[CMake] Put -Werror to CMAKE_CXX_FLAGS instead of using add_llvm_definitions()

add_definitions shouldn't really be used for compiler flags, and the variable
LLVM_DEFINITIONS is not appropriately used at the moment, e.g. it's not exported
to LLVMConfig.cmake


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203792 91177308-0d34-0410-b5e6-96231b3b80d8