[clang] Replace BACKEND_PACKAGE_STRING with LLVM_VERSION_STRING

420d7ccb 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.

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D136660