[Docs] Add/update release notes for D71913 (LTO WPD changes)

[tejohnson]

Summary

This adds documentation for the options added / changed by D71913, which
enabled aggressive WPD under LTO. The lld release notes already
mentioned it, but I expanded the note.

[MaskRay]

lld/docs/ReleaseNotes.rst

29

-fwhole-program-vtables -> add backquotes

llvm/docs/ReleaseNotes.rst

273

`-plugin-opt=whole-program-visibility`

[tejohnson]

Address comments

[hans]

Looks great, thanks!

[hans]

Please go ahead and commit to the branch.

[tejohnson]

In D86958#2274040, @hans wrote:

Please go ahead and commit to the branch.

Sorry, I thought I already did! Just pushed the commit.

[hans]

I see it now. Thanks!

[tejohnson]

This was committed awhile back (d3f1f588f902a968f102d6cdaf052674efc257aa), manually closing.

clang/docs/ReleaseNotes.rst

- The deprecated argument ``-f[no-]sanitize-recover`` has changed to mean
   ``-f[no-]sanitize-recover=all`` instead of
   ``-f[no-]sanitize-recover=undefined,integer`` and is no longer deprecated.
 - The argument to ``-f[no-]sanitize-trap=...`` is now optional and defaults to
   ``all``.
 - ``-fno-char8_t`` now disables the ``char8_t`` keyword, not just the use of
   ``char8_t`` as the character type of ``u8`` literals. This restores the
   Clang 8 behavior that regressed in Clang 9 and 10.
 - -print-targets has been added to print the registered targets.
+- ``-fwhole-program-vtables`` (along with ``-flto*``) now prepares all classes for possible whole program visibility if specified during the LTO link.
+  (`D71913 <https://reviews.llvm.org/D71913>`_)
 
 New Pragmas in Clang
 --------------------
 
 - ...
 
 Attribute Changes in Clang
 --------------------------

lld/docs/ReleaseNotes.rst

 Non-comprehensive list of changes in this release
 =================================================
 
 ELF Improvements
 ----------------
 
 * ``--lto-emit-asm`` is added to emit assembly output for debugging purposes.
   (`D77231 <https://reviews.llvm.org/D77231>`_)
-* ``--lto-whole-program-visibility`` is added to support LTO whole-program devirtualization.
+* ``--lto-whole-program-visibility`` is added to specify that classes have hidden LTO visibility in LTO and ThinLTO links of source files compiled with ``-fwhole-program-vtables``. See `LTOVisibility <https://clang.llvm.org/docs/LTOVisibility.html>`_ for details.
   (`D71913 <https://reviews.llvm.org/D71913>`_)
 * ``--print-archive-stats=`` is added to print the number of members and the number of fetched members for each archive.
   The feature is similar to GNU gold's ``--print-symbol-counts=``.
   (`D78983 <https://reviews.llvm.org/D78983>`_)
 * ``--shuffle-sections=`` is added to introduce randomization in the output to help reduce measurement bias and detect static initialization order fiasco.
   (`D74791 <https://reviews.llvm.org/D74791>`_)
   (`D74887 <https://reviews.llvm.org/D74887>`_)
 * ``--time-trace`` is added. It records a time trace file that can be viewed in

llvm/docs/ReleaseNotes.rst

 * LLVM now supports the debug entry values (DW_OP_entry_value) production for
   the x86, ARM, and AArch64 targets by default. Other targets can use
   the utility by using the experimental option ("-debug-entry-values").
   This is a debug info feature that allows debuggers to recover the value of
   optimized-out parameters by going up a stack frame and interpreting the values
   passed to the callee. The feature improves the debugging user experience when
   debugging optimized code.
 
+Changes to the Gold Plugin
+--------------------------
+
+* ``--plugin-opt=whole-program-visibility`` is added to specify that classes have hidden LTO visibility in LTO and ThinLTO links of source files compiled with ``-fwhole-program-vtables``. See `LTOVisibility <https://clang.llvm.org/docs/LTOVisibility.html>`_ for details.
+  (`D71913 <https://reviews.llvm.org/D71913>`_)
+
 Changes to the LLVM tools
 ---------------------------------
 
 * Added an option (--show-section-sizes) to llvm-dwarfdump to show the sizes
   of all debug sections within a file.
 
 * llvm-nm now implements the flag ``--special-syms`` and will filter out special
   symbols, i.e. mapping symbols on ARM and AArch64, by default. This matches