[XRay] Check in Clang whether XRay supports the target when -fxray-instrument is passed

Summary:
Added the code which explicitly emits an error in Clang in case
`-fxray-instrument` is passed, but XRay is not supported for the
selected target.

Reviewers: rsmith, aaron.ballman, rnk, dberris

Differential Revision: https://reviews.llvm.org/D24799

llvm-svn: 285266