Add lldb-mi deprecation to the release notes

[JDevlieghere]

Summary

Mention that LLVM 9 is the last release we'll ship lldb-mi as part of LLDB.

[teemperor]

+1

Nit pick: I think in 8.0.0 we actually gave LLDB it's own small section: https://releases.llvm.org/8.0.0/docs/ReleaseNotes.html#changes-to-lldb I guess we have some other stuff we could add to that section that it is not just "removed lldb-mi".

[JDevlieghere]

Move note into LLDB section.

[davide]

LGTM

[labath]

Random idea: Should we force the user to pass something like -DLLDB_TEMPORARILY_ENABLE_LLDB_MI to cmake to make sure it gets noticed that it is going away (I don't know how many people read release notes).

And yeah, I should write something about type units in the release notes...

llvm/docs/ReleaseNotes.rst

* The optimizer will now convert calls to ``memcmp`` into a calls to ``bcmp`` in
   some circumstances. Users who are building freestanding code (not depending on
   the platform's libc) without specifying ``-ffreestanding`` may need to either
   pass ``-fno-builtin-bcmp``, or provide a ``bcmp`` function.
 
 * Two new extension points, namely ``EP_FullLinkTimeOptimizationEarly`` and
   ``EP_FullLinkTimeOptimizationLast`` are available for plugins to specialize
   the legacy pass manager full LTO pipeline.
 
+* This release will be the last where ``lldb-mi`` is shipped as part of LLDB.
+  The tool will still be available in a `downstream repository on GitHub
+  <https://github.com/lldb-tools/lldb-mi>`_.
+
 .. 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
    -------------------