getCompilerVersion assumes that clang --version prints out a string like version [0-9\.]+.
If clang is built from trunk, the version line might look like clang version trunk (123abc).
Since there isn't any way of knowing by the commit id alone whether one commit is newer or older than another git commit (or clang version), assume that clang with a version id like this is very close to trunk. For example, any tests with @skipIf(compiler="clang", compiler_version=['<', '8']) should be run.