Revert "ios: Disable -Wgnu-folding-constant for .m files."

This reverts commit 0afe240a.

Reason for revert: The issue is fixed in iOS third_party code so the
change is no longer needed.

Original change's description:
> ios: Disable -Wgnu-folding-constant for .m files.
>
> Bug: 1143700
> Change-Id: If1110313fda223d83d17db5600f2041c3e1f6f32
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510229
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Commit-Queue: Hans Wennborg <hans@chromium.org>
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Reviewed-by: Hans Wennborg <hans@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#822644}

TBR=thakis@chromium.org,hans@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1143700
Change-Id: I3c1b2e57871b06bb21fed29fc4b4ecc6c77d9caa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2545025
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828393}

build/config/compiler/BUILD.gn

@@ -1558,11 +1558,6 @@ config("default_warnings") {
           "-Wno-implicit-fallthrough",
         ]
 
-        if (is_ios) {
-          # TODO(https://crbug.com/1143700): Clean up and re-enable.
-          cflags_objc = [ "-Wno-gnu-folding-constant" ]
-        }
-
         if (enable_wmax_tokens) {
           cflags += [ "-Wmax-tokens" ]
         } else {