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

[Adrian Prantl via llvm-commits]

adrian-prantl wrote:

This change looks good by itself, but we may need to additionally sneak in `-isysroot` if any system headers are needed by the tests.

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