message: Fixes reference of constant expression in core/kernels/function_op.cc.
  The reference was missing constexpr which caused trouble while instantiating templates.
diff: @@ -34,7 +34,8 @@ limitations under the License.
  namespace tensorflow {
  -static const char* const kGradientOp = FunctionLibraryDefinition::kGradientOp;
  +static constexpr const char* const kGradientOp =
  + FunctionLibraryDefinition::kGradientOp;
  ArgOp::ArgOp(OpKernelConstruction* ctx) : OpKernel(ctx) {
  OP_REQUIRES_OK(ctx, ctx->GetAttr("T", &dtype_));
  @@ -94,7 +95,6 @@ REGISTER_SYSTEM_KERNEL_BUILDER(Name(kDeviceRetOp).Device(DEVICE_CPU), RetvalOp);
  // is turned on.
  REGISTER_KERNEL_BUILDER(Name(kRetOp).Device(DEVICE_TPU_SYSTEM), RetvalOp);
  -
  #define REGISTER(type) \
  REGISTER_KERNEL_BUILDER( \
  Name(kArgOp).Device(DEVICE_GPU).TypeConstraint<type>("T"), ArgOp);
  @@ -203,7 +203,6 @@ REGISTER_KERNEL_BUILDER(Name("_ArrayToList")
  .TypeConstraint<int32>("T"),
  PassOn);
  -
  class SymbolicGradientOp : public AsyncOpKernel {
  public:
  explicit SymbolicGradientOp(OpKernelConstruction* ctx) : AsyncOpKernel(ctx) {}
  