420d7ccbac0f499a6ff9595bdbfa99cd3376df22 introduced BACKEND_PACKAGE_STRING to
replace PACKAGE_VERSION (llvm/Config/config.h) to support standalone builds.
This is used in the output of clang -cc1 -v.
Since llvm-config.h is available for both standalone and non-standalone builds,
we can just use LLVM_VERSION_STRING from llvm-config.h.
clang/cmake/modules/AddClang.cmake uses VERSION_STRING "${CLANG_VERSION} (${BACKEND_PACKAGE_STRING})".
Just simplify it to "${CLANG_VERSION}" so that we can remove the CMake
variable BACKEND_PACKAGE_STRING.