Add a presubmit rule for subspan(0u, n).

Using first(n) is shorter and can give better code in most cases.

Change-Id: I10747631af5f2d9d18fc57b5e25d49e99b83b377
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6288446
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1423283}

PRESUBMIT.py

@@ -1993,6 +1993,15 @@ _BANNED_CPP_FUNCTIONS: Sequence[BanRule] = (
           '^ios/',
         ),
     ),
+    BanRule(
+        pattern=r'subspan(0u,',
+        explanation=
+        ('Prefer first(n) over subspan(0u, n) as it is shorter, and the '
+         'compiler may have to emit a branch for the n == dynamic_extent '
+         'case of subspan().',
+        ),
+        treat_as_error=False,
+    ),
     BanRule(
         pattern=r'UNSAFE_TODO(',
         explanation=