Fixes:
```
/Users/ec2-user/jenkins/workspace/apple-llvm-project-pr-macos/branch-swift/release/6.2/llvm-project/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/invalid-vector/main.cpp:5:11: warning: inline namespace reopened as a non-inline namespace [-Winline-namespace-reopened-noninline]
5 | namespace __1 {
| ^
```
Drive-by: compile test as C++20 (in an attempt to fix another buildbot issue)
dfed3d235f