clang-tidy: reenable check with c++20-only guidance

Bug: 1406869
Change-Id: I7c873dc87be8cce97ae4559c21ada235047bb6e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4752497
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: George Burgess <gbiv@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1223722}

.clang-tidy

@@ -1,9 +1,6 @@
 ---
   # TODO(crbug.com/1282228): reenable google-readability-casting once it no
   # longer has as many false-positives.
-  # TODO(crbug.com/1406869): reenable modernize-use-default-member-init once
-  # C++20 is everywhere; it recommends using default member init for bit fields,
-  # which is C++20-only.
   Checks:          '-*,
                     bugprone-argument-comment,
                     bugprone-assert-side-effect,
@@ -27,6 +24,7 @@
                     modernize-replace-random-shuffle,
                     modernize-shrink-to-fit,
                     modernize-use-bool-literals,
+                    modernize-use-default-member-init,
                     modernize-use-emplace,
                     modernize-use-equals-default,
                     modernize-use-equals-delete,