[NFC][Offload] Move conformance test warning outside of function (llvm#152466)

`add_conformance_test` checks for libc and prints a warning if it is not
found. However, this warning ends up being printed once for each test,
spamming the cmake log. Moving it up to the folder cmake allows it to
be reported only once.

Author: RossBrunton

offload/unittests/CMakeLists.txt

@@ -104,11 +104,6 @@ function(add_conformance_test test_name)
 
   list(TRANSFORM ARGN PREPEND "${CMAKE_CURRENT_SOURCE_DIR}/" OUTPUT_VARIABLE files)
 
-  if(NOT TARGET libc)
-    message(WARNING "Cannot run conformance tests without the LLVM C library")
-    return()
-  endif()
-
   add_executable(${target_name} ${files})
   add_dependencies(${target_name} conformance_device_binaries)
   target_compile_definitions(${target_name}

offload/unittests/Conformance/tests/CMakeLists.txt

@@ -1,3 +1,8 @@
+if(NOT TARGET libc)
+    message(WARNING "Cannot run conformance tests without the LLVM C library")
+    return()
+endif()
+
 add_conformance_test(acosf AcosfTest.cpp)
 add_conformance_test(acoshf AcoshfTest.cpp)
 add_conformance_test(asinf AsinfTest.cpp)