[compiler-rt] Unit tests

I noticed that in compiler-rt there's a rule generated for the Fuzzer
unit tests (check-fuzzer-unit) but no rules generated for unit tests of
other components (xray, sanitizers, etc.). Is there any particular
reason this is so?

                            -David

I think this is a matter of the convention not being followed throughout. For the XRay side, I followed what was being done in the other libraries. It certainly makes sense to have a separate target just for the unit tests. There is a `check-xray` target which runs both the unit tests and the lit “end-to-end” tests, which has been sufficient for my day-to-day testing/development.

-- Dean