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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

@arsenm I see you've merged main into the branch, which has pinged me yet again on this patch, which I'll merge now; I'm just force-pushing a rebased version of this PR up to get the merge commit out of the branch history.

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