Mike Sartain, one of the original Valve Linux developers who I know from my time at Valve, decided to write to clear up misconceptions about why Valve/RAD Game Tools is interested in working on LLDB.
Long story short, the Linux support has traditionally been quite poor, and lots of progress has been made in recent months, but much more work is needed. Sartain says that it will still be a lot of work to do before LLDB is capable of debugging Left 4 Dead or Team Fortress 2. LLDB right now on Linux can't even do a 32-bit debug of a simple printf statement.
With LLDB right now, 32-bit Linux debugging is problematic, DWARF4 symbols don't work, several test cases fail (~33 of 262), a tendency to hang, and there's no remote debugging support, among other shortcomings.
More details can be found in Mike's new blog post.
On a slightly unrelated note, good news is that I'm back to talking with Valve about addressing some Steam Linux issues to allow for automated benchmarking so hopefully that will be resolved soon and be fully supported by the Phoronix Test Suite.