On non Debian-based systems libclang is mostly installed as
libclang.so.<version>, evading detection by
clang-${_LIBCLANG_FIND_VERSION}.0. Instead of specyfing and maintaing
a list of library names I imported FindClang from KDevelop, which
finds the correct library here and, one would assume, also works on
Debian and distros based on it.
I didn't use ClangConfig.cmake from upstream because it doesn't allow
discovery of a certain version.