Begin sketching out the Clang release notes. The plan is to link to

these more detailed notes from the primary LLVM release notes for Clang.
This gives us a nice place to flesh out in plenty of detail the major
changes that have happened in Clang land since 2.9.

I've outlined a very rough structure based on the LLVM release notes
structure and what seems like useful divisions in the Clang landscape
(e.g., language-specific stuff is relevant to a narrower audience).

I'll be first converting my brain-dump-ish notes from the commit logs,
and then cleaning here. Suggestions on structure welcome. Typo
corrections, spelling fixes (oh how I'll need them), all welcome; just
commit away.

llvm-svn: 145233