[HIP, test] Fix use of undef FileCheck var

Clang test CodeGenCUDA/kernel-stub-name.cu uses never defined DKERN
variable in a CHECK-NOT directive. This commit replace the variable by a
regex, thereby avoiding the issue.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D99832

Author: Thomas Preud'homme

clang/test/CodeGenCUDA/kernel-stub-name.cu

@@ -126,4 +126,4 @@ void fun5() {
 // CHECK: call{{.*}}@__hipRegisterFunction{{.*}}@[[HCKERN]]{{.*}}@[[CKERN]]
 // CHECK: call{{.*}}@__hipRegisterFunction{{.*}}@[[HNSKERN]]{{.*}}@[[NSKERN]]
 // CHECK: call{{.*}}@__hipRegisterFunction{{.*}}@[[HTKERN]]{{.*}}@[[TKERN]]
-// CHECK-NOT: call{{.*}}@__hipRegisterFunction{{.*}}@[[HDKERN]]{{.*}}@[[DKERN]]
+// CHECK-NOT: call{{.*}}@__hipRegisterFunction{{.*}}@[[HDKERN]]{{.*}}@{{[0-9]*}}