Enable -Wc++20-designator

Bug: dawn:394
Change-Id: I03f0c779e4fb3f0a32d171e986de885c857fe782
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/20580
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/common/BUILD.gn b/src/common/BUILD.gn
index 4ef4b70..e0cea4f 100644
--- a/src/common/BUILD.gn
+++ b/src/common/BUILD.gn
@@ -114,7 +114,6 @@
 
     # Flags for -pedantic warnings that haven't been fixed yet.
     cflags += [
-      "-Wno-c++20-designator",
       "-Wno-four-char-constants",
       "-Wno-gnu-anonymous-struct",
       "-Wno-gnu-zero-variadic-macro-arguments",