[gn] no propagating diagnostic flags (#12219)

This commit moves the diagnostic flags into the toolchain itself,
preventing these flags being propagated to OpenThread dependents.

script/check-gn-build

@@ -34,23 +34,23 @@
 set -e
 set -x
 
-main()
+check_build()
 {
-    # Check GN build for OT1.1
     rm gn-out -r || true
-    gn gen --check gn-out
-    gn args gn-out --list
-    ninja -C gn-out
-    test -f gn-out/obj/src/core/libopenthread-ftd.a
-
-    # Check GN build for OT1.4
-    rm gn-out -r || true
-    mkdir gn-out
-    echo 'openthread_config_thread_version = "1.4"' >gn-out/args.gn
-    gn gen --check gn-out
+    gn gen --check gn-out "$@"
     gn args gn-out --list
     ninja -C gn-out
     test -f gn-out/obj/src/core/libopenthread-ftd.a
 }
 
+main()
+{
+    for thread_version in 1.4 1.1; do
+        for use_clang in true false; do
+            check_build --args="use_clang=$use_clang openthread_config_thread_version=\"$thread_version\""
+        done
+    done
+
+}
+
 main "$@"