Given that Apple are the major sponsor of LLVM and they're not exactly cash poor, I think it's reasonable to conclude that being able to throw money and people at LLVM development doesn't explain why it's still much slower than the Go compiler toolchain. In hindsight the Go team made the right call to use their own toolchain.
As enneff alludes, Ken Thompson's antipathy towards C++ is well documented: https://bryanpendleton.blogspot.com/2009/12/coders-at-work-k...