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

[Shubham Sandeep Rastogi via llvm-commits]

rastogishubham wrote:

@SLTozer We managed to track down the real issue and the change is in https://github.com/llvm/llvm-project/pull/66299

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