Add driver test, fix it the way Mike suggested

Author: Erich Keane

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5962,9 +5962,6 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (Args.hasFlag(options::OPT_fsycl, options::OPT_fno_sycl, false))
     Args.AddLastArg(CmdArgs, options::OPT_sycl_std_EQ);
 
-  // Forward -fsycl-default-sub-group-size if in SYCL mode.
-  if (Args.hasFlag(options::OPT_fsycl, options::OPT_fno_sycl, false))
-    Args.AddLastArg(CmdArgs, options::OPT_fsycl_default_sub_group_size);
 
   // Forward -fsycl-instrument-device-code option to cc1. This option can only
   // be used with spir triple.
@@ -6682,6 +6679,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
         HeaderOpt.append(Header);
         CmdArgs.push_back(Args.MakeArgString(HeaderOpt));
       }
+
+      // Forward -fsycl-default-sub-group-size if in SYCL mode.
+      Args.AddLastArg(CmdArgs, options::OPT_fsycl_default_sub_group_size);
     }
 
     if (Args.hasArg(options::OPT_fsycl_unnamed_lambda))

clang/test/Driver/sycl-sub-group-size.cpp

@@ -0,0 +1,8 @@
+// RUN: %clang -### -fsycl %s 2>&1 | FileCheck %s -check-prefix=CHECK-DEFAULT
+// CHECK-DEFAULT-NOT: "fsycl-default-sub-group-size"
+
+// RUN: %clang -### -fsycl -fsycl-default-sub-group-size=primary %s 2>&1 | FileCheck %s -check-prefix=PRIM
+// PRIM: "-fsycl-default-sub-group-size" "primary"
+
+// RUN: %clang -### -fsycl -fsycl-default-sub-group-size=10 %s 2>&1 | FileCheck %s -check-prefix=TEN
+// TEN: "-fsycl-default-sub-group-size" "10"