[llvm] [Offload] Move RPC server handling to a dedicated thread (PR #112988)

[Shilei Tian via llvm-commits]

shiltian wrote:

> I think right now this will always create the thread, since I figured it's fine if it just goes to sleep immediately and is never woken up by a kernel launch that needs an RPC server, but I could make it only do that if we need the RPC server at all, then it's a pay for what you use scenario.

I think that is much more preferable if we really want to use extra thread to do the work.

https://github.com/llvm/llvm-project/pull/112988