[XRay] [compiler-rt] [NFC] Annotate unused variables for the compiler.

Summary:
Use a common definition of a "this variable is unused" annotation for useless
variables only present for their lambda global initializers, to silence gcc's
warning.

Reviewers: dberris

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29860

llvm-svn: 296449