[clang] [HIP] Move HIP to the new driver by default (PR #123359)

[via cfe-commits]

b-sumner wrote:

> I'd like to avoid modifying this struct to avoid breaking ABI with OpenMP. Perhaps I should make `addr` a pointer to a struct and just GEP the two values.

Are we trying to jam a square HIP peg into a round OpenMP hole, or are we truly wanting to move to a language neutral base?  If the latter, why don't we fix the base if it isn't sufficient?

https://github.com/llvm/llvm-project/pull/123359