D3D12: only lazy clear OutputAttachment textures with render target ops

Previously, lazy clearing always added DEPTH_STENCIL or RENDER_TARGET to
textures because we cleared using ClearDepthStencilView or
ClearRenderTargetView. Now, we're able to clear using copies.

This also allows textures to actually use the small resource heap
placement optimization. Doing so generates debug layer warnings when the
small alignment is first tried but rejected. This CL silences those
warnings.

Bug: dawn:145
Change-Id: Id385846536b337cddcfdadc5739561c7adc30c8c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/26840
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/tests/end2end/TextureZeroInitTests.cpp

@@ -30,6 +30,7 @@
         }                                                                                     \
     } while (0)
 
+// TODO(natlee@microsoft.com): test compressed textures are cleared
 class TextureZeroInitTest : public DawnTest {
   protected:
     void SetUp() override {