Add a link from the LLVM release notes to the Clang release notes.

Add a link from the LLVM release notes to the Clang release notes.
I suspect we could profitably remove/move some of the bullet points
under Clang here to the Clang notes in order to keep things clean on
both sides. Unless I hear objections I'll start doing that once folks
have read over the Clang notes a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145340 91177308-0d34-0410-b5e6-96231b3b80d8