Add XRay flags to Clang. We implement two flags to control the XRay behaviour:

-fxray-instrument: enables XRay annotation of IR
-fxray-instruction-threshold: configures the threshold for function size (looking at IR instructions), and allow LLVM to decide whether to add the nop sleds later on in the process.

Also implements the related xray_always_instrument and xray_never_instrument function attributes.

Patch by Dean Michael Berris.

llvm-svn: 275330