[XRay] Probe for CPU features that XRay needs

Summary:
In llvm.org/PR31756 it's pointed out that sometimes rdtscp isn't
available. We fix it here by checking first whether it's availble before
installing the logging handler. In future commits we can have
alternative implementations, maybe working around some of the
constraints on some systems.

This change enables us to make that determination, but report an error
instead when the features aren't available.

Reviewers: sdardis, javed.absar, rSerge

Subscribers: pelikan, llvm-commits

Differential Revision: https://reviews.llvm.org/D29438

llvm-svn: 293870