setup-build: gn sets use_clang_modules=false

When using clang modules, the standalone build fails
when compiling abseil-cpp/absl/strings/coord.cc with
errr 'module std is needed but has not been provided' when
trying to #include <algorithm>

Change-Id: I127fc37f2b88e2da990a9cc9a542fb136a6a6964
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/266674
Reviewed-by: Peter McNeeley <petermcneeley@google.com>

tools/setup-build

diff --git a/tools/setup-build b/tools/setup-build
index f359bac..b5fb84c 100755
--- a/tools/setup-build
+++ b/tools/setup-build
@@ -230,6 +230,7 @@
     if grep 'dawn_node.*True' .gclient >/dev/null; then
         GN_ARGS+=" dawn_build_node_bindings=true"
     fi
+    GN_ARGS+=" use_clang_modules=false"
     if [[ -z "$FORCE" && -f "$ROOT_DIR/out/$BUILD_DIR/args.gn" ]]; then
       generate "gn" "gen" $GN_OPTIONS "out/active" # keep existing args
     else