[llvm-branch-commits] [NFCI] [FlowSensitive] [StatusOr] use c++20 for test (PR #182603)

[Rohan Jacob-Rao via llvm-branch-commits]

https://github.com/rohanjr approved this pull request.

Agreed elsewhere that testing with older versions is overkill for now so we're fine to only upgrade the language version for tests.

https://github.com/llvm/llvm-project/pull/182603