[clang] Mention -Werror changes revived for Clang 16

[thesamesam]

Summary

-Wimplicit-function-declaration and -Wimplicit-int become errors
by default in Clang 16 (originally in 15, but we reverted it in 15.0.1).

Mention it in the release notes like we did originally for Clang 15.

See https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213 for more context.

[thesamesam]

I could use identical phrasing to -Wincompatible-function-pointer-types below (reference how to downgrade the error) but I don't think there's a need.

[aaron.ballman]

Whoops, I thought I had already added those release notes, but I didn't! Thank you for catching that! LGTM

[mgorny]

Thanks for doing this.

clang/docs/ReleaseNotes.rst

- Clang will now correctly diagnose as ill-formed a constant expression where an
 
   Due to the extended period of time this bug was present in major C++
   implementations (including Clang), this error has the ability to be
   downgraded into a warning (via: ``-Wno-error=enum-constexpr-conversion``) to
   provide a transition period for users. This diagnostic is expected to turn
   into an error-only diagnostic in the next Clang release. Fixes
   `Issue 50055 <https://github.com/llvm/llvm-project/issues/50055>`_.
 
+- The ``-Wimplicit-function-declaration`` and ``-Wimplicit-int`` warnings
+  now default to an error in C99, C11, and C17. As of C2x,
+  support for implicit function declarations and implicit int has been removed,
+  and the warning options will have no effect. Specifying ``-Wimplicit-int`` in
+  C89 mode will now issue warnings instead of being a noop.
+
+  **NOTE**: We recommend that projects using configure scripts verify that the
+  results do not change before/after setting
+  ``-Werror=implicit-function-declarations`` or ``-Wimplicit-int`` to avoid
+  incompatibility with Clang 16.
+
 - ``-Wincompatible-function-pointer-types`` now defaults to an error in all C
   language modes. It may be downgraded to a warning with
   ``-Wno-error=incompatible-function-pointer-types`` or disabled entirely with
   ``-Wno-incompatible-function-pointer-types``.
 
   **NOTE:** We recommend that projects using configure scripts verify that the
   results do not change before/after setting
   ``-Werror=incompatible-function-pointer-types`` to avoid incompatibility with