[clang] [Sema][Test] Add c++17 standard specifier to diagnostic test. (PR #222907)

[Tom Weaver via cfe-commits]

TomWeaver18 wrote:

Sure, I can do that, wasn't sure if the extra diagnostics were appropriate for this area to begin with. I'll get that sorted ASAP. thanks for the review! :)

https://github.com/llvm/llvm-project/pull/222907