Revert r310489 and follow-up commits r310505, r310519, r310537 and r310549

Commit r310489 caused 'openmp-offload.c' test failures on Darwin and other
platforms:
http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-incremental_check/39230/testReport/junit/Clang/Driver/openmp_offload_c/

The follow-up commits tried to fix the test, but the test is still failing.

llvm-svn: 310580
diff --git a/clang/lib/Driver/ToolChains/Cuda.cpp b/clang/lib/Driver/ToolChains/Cuda.cpp
index a0bbd52..643c340 100644
--- a/clang/lib/Driver/ToolChains/Cuda.cpp
+++ b/clang/lib/Driver/ToolChains/Cuda.cpp
@@ -484,13 +484,7 @@
   // than LLVM defaults to. Use PTX4.2 which is the PTX version that
   // came with CUDA-7.0.
   CC1Args.push_back("-target-feature");
-
-  if (DeviceOffloadingKind == Action::OFK_OpenMP)
-    CC1Args.push_back(
-        DriverArgs.getLastArgValue(options::OPT_fopenmp_ptx_EQ,
-                                   "+ptx42").data());
-  else
-    CC1Args.push_back("+ptx42");
+  CC1Args.push_back("+ptx42");
 }
 
 void CudaToolChain::AddCudaIncludeArgs(const ArgList &DriverArgs,
@@ -533,14 +527,10 @@
     }
 
     StringRef Arch = DAL->getLastArgValue(options::OPT_march_EQ);
-    if (Arch.empty()) {
-      // Default compute capability for CUDA toolchain is the
-      // lowest compute capability supported by the installed
-      // CUDA version.
+    if (Arch.empty())
+      // Default compute capability for CUDA toolchain is sm_20.
       DAL->AddJoinedArg(nullptr,
-          Opts.getOption(options::OPT_march_EQ),
-          CudaInstallation.getLowestExistingArch());
-    }
+          Opts.getOption(options::OPT_march_EQ), "sm_20");
 
     return DAL;
   }