[llvm] [Dexter] Replace clang with clang++ in various cross project tests (PR #65987)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> Since the issue had nothing to do with clang++, is this change needed? @SLTozer @adrian-prantl

It's not needed to make the tests pass, but we should probably be invoking the right driver for tests where reasonable, so I think it's still worth landing now that it's done.

https://github.com/llvm/llvm-project/pull/65987