Updated the LLDB status page to reflect the recent addition of multi-threaded support.

Reworked the download information on lldb.llvm.org:
- svn copy of download.html to source.html with information on source-code access
- new download.html with links to download nightly builds and Debian releases
- updated the sidebar to reflect these changes



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183547 91177308-0d34-0410-b5e6-96231b3b80d8