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

[Tom Weaver via cfe-commits]

TomWeaver18 wrote:

I tried added the original author of 1ec7a13fb238 but for some reason GH wouldn't find them in the list.

Hope you're ok with reviewing this @cor3ntin and thanks in advance! :)

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