[Release Notes] Add release note for the new LLVM_DEBUG

[Nicola]

Summary

This is to provide a way to migrate from the old DEBUG macro to the new one.

[dsanders]

LGTM with a note about clang-format

docs/ReleaseNotes.rst

97

We should probably mention that we also ran clang-format afterwards and provide a command for that. If someone needs to backport it to older branches then they'll want to use it to minimize the differences between the branches.

docs/ReleaseNotes.rst

     clang -O1 ftrunc.c -fsanitize=float-cast-overflow ; ./a.out 
     ftrunc.c:5:15: runtime error: 4.29497e+09 is outside the range of representable values of type 'int'
     junk in the ftrunc: 0.000000
 
 * ``LLVM_ON_WIN32`` is no longer set by ``llvm/Config/config.h`` and
   ``llvm/Config/llvm-config.h``.  If you used this macro, use the compiler-set
   ``_WIN32`` instead which is set exactly when ``LLVM_ON_WIN32`` used to be set.
 
+* The ``DEBUG`` macro has been renamed to ``LLVM_DEBUG``. The interface remains
+  the same.  If you used this macro you need to migrate to the new one, you can
+  do this with the following bash command:
+
+.. code-block:: bash
+
+    git grep -l 'DEBUG' | xargs perl -pi -e 's/\bDEBUG\s?\(/LLVM_DEBUG(/g'
+
 * Note..
 
 .. NOTE
    If you would like to document a larger change, then you can add a
    subsection about it right here. You can copy the following boilerplate
    and un-indent it (the indentation causes it to be inside this comment).
 
    Special New Feature