[XRay][compiler-rt] Do not print the warning when the binary is not XRay instrumented.

Summary:
Currently when the XRay runtime is linked into a binary that doesn't
have the instrumentation map, we print a warning unconditionally. This
change attempts to make this behaviour more quiet.

Reviewers: kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35789

llvm-svn: 309534