Added a buildbot script that automatically checks

out the latest LLDB, LLVM and Clang and makes sure
they work okay together.  The buildbot is currently
Mac OS X only because it uses xcodebuild.

Right now, the portion that runs the LLDB test
suite is commented out because of code-signing
problems (specifically, on Mac OS 10.7 a UI dialog
appears asking for the user's permission to attach
to the inferior process).

You can use the buildbot like this:

./scripts/buildbot.py -b /tmp/lldb-build -l /tmp/lldb.log

/tmp/lldb-build and /tmp/lldb.log should not exist
when the script is run.

/tmp/lldb-build is a temporary directory and is
removed at the end of a normal execution.

/tmp/lldb.log is a log file that sticks around.

The buildbot does not require built versions of anything;
it will do the building itself.

llvm-svn: 142006