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/DawnTest.cpp

@@ -610,6 +610,14 @@ bool DawnTestBase::IsDawnValidationSkipped() const {
     return gTestEnv->IsDawnValidationSkipped();
 }
 
+bool DawnTestBase::HasWGSL() const {
+#ifdef DAWN_ENABLE_WGSL
+    return true;
+#else
+    return false;
+#endif
+}
+
 bool DawnTestBase::IsAsan() const {
 #if defined(ADDRESS_SANITIZER)
     return true;