[XRay][compiler-rt] Disable XRay instrumentation of the XRay runtime.

Summary:
Adds a CMake check for whether the compiler used to build the XRay
library supports XRay-instrumentation. If the compiler we're using does
support the `-fxray-instrument` flag (i.e. recently-built Clang), we
define the XRAY_NEVER_INSTRUMENT macro that then makes sure that the
XRay runtime functions never get XRay-instrumented.

This prevents potential weirdness involved with building the XRay
library with a Clang that supports XRay-instrumentation, and is
attempting to XRay-instrument the build of compiler-rt.

Reviewers: majnemer, rSerge, echristo

Subscribers: mehdi_amini, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D26597

llvm-svn: 287068