Don't connect to backends that aren't needed

Change backend connection in Instance.cpp to store a bitset of
backends that have been connected. This lets us only connect to a
single backend if AdapterDiscoveryOptions are passed explicitly,
and track which connections have/have not been made. Later, we can
connect to the rest of the backends if more are requested.

This is part of some improvements to the existing code so we can
selectively discover adapters and control discovery of the
high-performance, low-power, and fallback WebGPU adapters.

Bug: chromium:1266550
Change-Id: Iceb0d3f71751f5aac6218996ace3cf89deda8a29
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/69521
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

generator/templates/dawn_native/dawn_platform.h

@@ -59,6 +59,16 @@ namespace dawn_native {
             return reinterpret_cast<{{as_cppType(type.name)}}Base*>(rhs);
         }
     {% endfor %}
+
+    template <typename T>
+    struct EnumCount;
+
+    {% for e in by_category["enum"] if e.contiguousFromZero %}
+        template<>
+        struct EnumCount<wgpu::{{as_cppType(e.name)}}> {
+            static constexpr uint32_t value = {{len(e.values)}};
+        };
+    {% endfor %}
 }
 
 #endif  // DAWNNATIVE_DAWN_PLATFORM_AUTOGEN_H_