Fix "fallthrough annotation in unreachable code" in release d3d builds

The same issue I ran into in https://crrev.com/c/6231111 but on d3d

Bug: angleproject:394129077
Change-Id: I90faa01fa66c514162a1d5e4cbe56a2bfd85085e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6237859
Reviewed-by: Cody Northrop <[email protected]>
Commit-Queue: Cody Northrop <[email protected]>
Auto-Submit: Roman Lavrov <[email protected]>

Author: romanl-g

src/libANGLE/renderer/d3d/d3d11/ResourceManager11.cpp

@@ -314,7 +314,7 @@ DXGI_FORMAT GetTypedColorFormatForClearing(DXGI_FORMAT dxgiFormat)
         case DXGI_FORMAT_R24_UNORM_X8_TYPELESS:
         case DXGI_FORMAT_X24_TYPELESS_G8_UINT:
             UNREACHABLE();
-            [[fallthrough]];
+            return dxgiFormat;
         default:
             return dxgiFormat;
     }