[CUDA] Require libdevice only if needed

[Hahnfeld]

Summary

If the user passes -nocudalib, we can live without it being present.
Simplify the code by just checking whether LibDeviceMap is empty.

[tra]

The change could use a test.

[Hahnfeld]

Fix one more condition that checks for nvvm/libdevice and add a test.

[tra]

Looks good. Thank you.

lib/Driver/ToolChains/Cuda.cpp

if (Version == CudaVersion::CUDA_90) {
             LibDeviceMap["sm_50"] = FilePath;
             LibDeviceMap["sm_52"] = FilePath;
             LibDeviceMap["sm_53"] = FilePath;
           }
         }
       }
     }
 
-    // This code prevents IsValid from being set when
-    // no libdevice has been found.
-    bool allEmpty = true;
-    std::string LibDeviceFile;
-    for (auto key : LibDeviceMap.keys()) {
-      LibDeviceFile = LibDeviceMap.lookup(key);
-      if (!LibDeviceFile.empty())
-        allEmpty = false;
-    }
-
-    if (allEmpty)
+    // Check that we have found at least one libdevice that we can link in if
+    // -nocudalib hasn't been specified.
+    if (LibDeviceMap.empty() && !Args.hasArg(options::OPT_nocudalib))
       continue;
 
     IsValid = true;
     break;
   }
 }
 
 void CudaInstallationDetector::AddCudaIncludeArgs(