ShaderModule: Add support for multiple entryPoints

Also adds validation tests that reflection data is correctly computed by
entryPoint, and end2end tests that using a shader module with multiple
entryPoints works correctly.

Bug: dawn:216
Change-Id: Id2936bb220d4480872a68624996e4c42452a507d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/28244
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/common/BUILD.gn

@@ -75,6 +75,10 @@ config("dawn_internal") {
     defines += [ "DAWN_ENABLE_BACKEND_VULKAN" ]
   }
 
+  if (dawn_enable_wgsl) {
+    defines += [ "DAWN_ENABLE_WGSL" ]
+  }
+
   if (dawn_use_x11) {
     defines += [ "DAWN_USE_X11" ]
   }