[xray] Don't try to run XRay unit tests on Darwin

[kubamracek]

Summary

Currently, the lit produces a warning:

llvm/utils/lit/lit/discovery.py:244: warning: input './projects/compiler-rt/test/xray/Unit' contained no tests

Let's get rid of it.

[dberris]

LGTM

test/xray/CMakeLists.txt

foreach(arch ${XRAY_TEST_ARCH})
     configure_lit_site_cfg(
       ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
       ${CMAKE_CURRENT_BINARY_DIR}/${CONFIG_NAME}/lit.site.cfg)
     list(APPEND XRAY_TESTSUITES ${CMAKE_CURRENT_BINARY_DIR}/${CONFIG_NAME})
   endforeach()
 
   # Add unit tests.
   if(COMPILER_RT_INCLUDE_TESTS)
+    if(UNIX AND NOT APPLE)
       configure_lit_site_cfg(
         ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.in
         ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg)
       list(APPEND XRAY_TEST_DEPS XRayUnitTests)
       list(APPEND XRAY_TESTSUITES ${CMAKE_CURRENT_BINARY_DIR}/Unit)
     endif()
   endif()
+endif()
 
 add_lit_testsuite(check-xray "Running the XRay tests"
   ${XRAY_TESTSUITES}
   DEPENDS ${XRAY_TEST_DEPS})
 set_target_properties(check-xray PROPERTIES FOLDER "Compiler-RT Misc")