Don't look for Clang's builtin dir in "unclean" version subdir

Since 6da2caac KDevelop looks for
Clang's builtin dir in the ${CLANG_VERSION_CLEAN} subdirectory in
addition to the ${CLANG_VERSION} subdirectory. As
ClangHelpers::clangBuiltinIncludePath() does not consider a not-clean
version subdirectory, CMake code does not need to consider it either.