Implement getBindGroupLayout

This patch makes the |layout| member of the Render|ComputePipelineDescriptor
optional. If it is not provided, a default layout is created from the
ShaderModules provided and used to replace the layout in the descriptor.

Then, pipeline.GetBindGroupLayout may be called to get the existing, or
the computed bind group layout. If no bind group layout exists at the
provided index, an empty bind group layout is returned.

Bug: dawn:276
Change-Id: I276ed0296a2f1f2d8131fa906a4aefe85d75b3a7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13741
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jiawei Shao <jiawei.shao@intel.com>

generator/templates/webgpu_cpp.h

@@ -108,6 +108,13 @@ namespace wgpu {
             return static_cast<Derived&>(*this);
         }
 
+        bool operator==(std::nullptr_t) const {
+            return mHandle == nullptr;
+        }
+        bool operator!=(std::nullptr_t) const {
+            return mHandle != nullptr;
+        }
+
         explicit operator bool() const {
             return mHandle != nullptr;
         }