Skip tests that use 'llvm_xray' for standalone builds.

Summary:
Tests that generate output with compiler-rt and verify it with the llvm_xray
command (built from the llvm tree) are extremely convenient, but compiler-rt
can be built out of tree and llvm_xray is not built for every target.

This change intends to disable tests for out of tree builds, but does nothing
to detect whether llvm_xray can be found elsewhere on the path, is fresh enough,
or is part of a build target for the in tree build.

Tested:
  Tested that this didn't break check-xray. Haven't reproduced bots or standalone
  builds.

Reviewers: dberris, kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32150

llvm-svn: 300716