[lldb/test] Support git commit version ids for clang.

`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.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D85248