ReleaseNotes.rst: a few entries from Rafael

  Core.h so nothing should change for projects directly including the headers,

  but transitive dependencies may be affected.

* llvm-ar now suports thin archives.

* llvm doesn't produce .data.rel.ro.local or .data.rel sections anymore.

* aliases to available_externally globals are now rejected by the verifier.

* the IR Linker has been split into IRMover that moves bits from one module to

  another and Linker proper that decides what to link.

* Support for dematerializing has been dropped.

.. NOTE

   For small 1-3 sentence descriptions, just add an entry at the end of

   this list. If your description won't fit comfortably in one bullet