Fix GCC builds

BUG=tint:778

Change-Id: If06e453b6fd58b8c74bed996532b3b9f9f7804ee
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/50044
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/resolver/resolver.cc

diff --git a/src/resolver/resolver.cc b/src/resolver/resolver.cc
index e098271..603f3dd 100644
--- a/src/resolver/resolver.cc
+++ b/src/resolver/resolver.cc
@@ -397,7 +397,7 @@
       }
       return nullptr;
     }
-    if (auto* t = ty->As<ast::ExternalTexture>()) {
+    if (ty->As<ast::ExternalTexture>()) {
       return builder_->create<sem::ExternalTexture>();
     }
     if (auto* t = ty->As<ast::TypeName>()) {