[Driver][SYCL] Improve -std setting behaviors for Windows (#2461)

When using -std:arg on Windows, the std setting was not being applied
to the device compilation.  Make needed adjustements to allow this
which is due to the -std option differences in regards to Windows.

Author: mdtoguchi

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4056,10 +4056,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   // device toolchain.
   bool UseSYCLTriple = IsSYCLDevice && (!IsSYCL || IsSYCLOffloadDevice);
 
-  // Adjust IsWindowsXYZ for CUDA/HIP compilations.  Even when compiling in
+  // Adjust IsWindowsXYZ for CUDA/HIP/SYCL compilations.  Even when compiling in
   // device mode (i.e., getToolchain().getTriple() is NVPTX/AMDGCN, not
   // Windows), we need to pass Windows-specific flags to cc1.
-  if (IsCuda || IsHIP)
+  if (IsCuda || IsHIP || IsSYCL)
     IsWindowsMSVC |= AuxTriple && AuxTriple->isWindowsMSVCEnvironment();
 
   // C++ is not supported for IAMCU.
@@ -5174,7 +5174,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                                 /*Joined=*/true);
     else if (IsWindowsMSVC)
       ImplyVCPPCXXVer = true;
-    else if (IsSYCL)
+
+    if (IsSYCL && types::isCXX(InputType) &&
+        !Args.hasArg(options::OPT__SLASH_std))
       // For DPC++, we default to -std=c++17 for all compilations.  Use of -std
       // on the command line will override.
       CmdArgs.push_back("-std=c++17");
@@ -5758,12 +5760,11 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     }
 
     if (LanguageStandard.empty()) {
-      if (IsMSVC2015Compatible)
-        if (IsSYCL)
-          // For DPC++, C++17 is the default.
-          LanguageStandard = "-std=c++17";
-        else
-          LanguageStandard = "-std=c++14";
+      if (IsSYCL)
+        // For DPC++, C++17 is the default.
+        LanguageStandard = "-std=c++17";
+      else if (IsMSVC2015Compatible)
+        LanguageStandard = "-std=c++14";
       else
         LanguageStandard = "-std=c++11";
     }

clang/test/Driver/sycl-offload.c

@@ -898,7 +898,9 @@
 // -std=c++17 override check
 // RUN: %clangxx -### -c -fsycl -std=c++14 -xc++ %s 2>&1 | FileCheck -check-prefix=CHECK-STD-OVR %s
 // RUN: %clang_cl -### -c -fsycl /std:c++14 -TP %s 2>&1 | FileCheck -check-prefix=CHECK-STD-OVR %s
-// CHECK-STD-OVR: clang{{.*}} "-std=c++14"
+// CHECK-STD-OVR: clang{{.*}} "-emit-llvm-bc" {{.*}} "-std=c++14"
+// CHECK-STD-OVR: clang{{.*}} "-fsyntax-only" {{.*}} "-std=c++14"
+// CHECK-STD-OVR: clang{{.*}} "-emit-obj" {{.*}} "-std=c++14"
 // CHECK-STD-OVR-NOT: clang{{.*}} "-std=c++17"
 
 // TODO: SYCL specific fail - analyze and enable