Fixup shadow variable.

This CL fixes up a shadow variable which causes compilation issues
on OSX.

Change-Id: I750423630cc2882d176c5ef304e1e0b4bef133a6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/108040
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: Ben Clayton <bclayton@chromium.org>

src/tint/cmd/main.cc

diff --git a/src/tint/cmd/main.cc b/src/tint/cmd/main.cc
index 73a90d8..f27e643 100644
--- a/src/tint/cmd/main.cc
+++ b/src/tint/cmd/main.cc
@@ -1081,9 +1081,9 @@
              // Set of all the group-0 bindings in use.
              std::unordered_set<uint32_t> group0_bindings_in_use;
              auto allocate_binding = [&] {
-                 for (uint32_t i = 0;; i++) {
-                     auto binding = tint::transform::BindingPoint{0u, i};
-                     if (group0_bindings_in_use.emplace(i).second) {
+                 for (uint32_t idx = 0;; idx++) {
+                     auto binding = tint::transform::BindingPoint{0u, idx};
+                     if (group0_bindings_in_use.emplace(idx).second) {
                          return binding;
                      }
                  }