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/tests/unittests/validation/ValidationTest.cpp

@@ -102,6 +102,14 @@ void ValidationTest::WaitForAllOperations(const wgpu::Device& device) const {
     device.Tick();
 }
 
+bool ValidationTest::HasWGSL() const {
+#ifdef DAWN_ENABLE_WGSL
+    return true;
+#else
+    return false;
+#endif
+}
+
 // static
 void ValidationTest::OnDeviceError(WGPUErrorType type, const char* message, void* userdata) {
     ASSERT(type != WGPUErrorType_NoError);