So you saw that talk at WWDC as well? Every engineer from Apple that I've spoken to and asked about this has gone "What?! Did he really say that? Ignore it. We don't recommend that. Not at all."
Regardless of what they say, Apple is still claiming new two-character prefixes, including the one I've been using for years.
You work on LLVM? Props to you, if so. Thats an awesome project. However, if you work on gcc, I haven't used that in over a year :-)
I'm a big fan of LLVM, but no, I've done nothing with it. That said, if you're using clang for your production builds, you're a much braver soul than me.
There are still plenty of codegen issues, especially in the new iPhone/ARM support. There's a reason Apple is still using GCC.