Fix test added in 849de61 to work when the compiler defaults to C++20. (#193361)
Our downstream compiler defaults to C++20 mode which causes the test
added in #189775 to fail due to some additional notes that are emitted
which causes the `-verify` steps to fail.
```
# .---command stderr------------
# | error: 'cpp20-note' diagnostics seen but not expected:
# | File /home/dyung/src/git/merge/clang/test/APINotes/unsafe-buffer-usage.cpp Line 59: pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions
# | File /home/dyung/src/git/merge/clang/test/APINotes/unsafe-buffer-usage.cpp Line 64: pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions
# | File /home/dyung/src/git/merge/clang/test/APINotes/unsafe-buffer-usage.cpp Line 65: pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions
# | File /home/dyung/src/git/merge/clang/test/APINotes/unsafe-buffer-usage.cpp Line 66: pass -fsafe-buffer-usage-suggestions to receive code hardening suggestions
# | 4 errors generated.
# `-----------------------------
```
This change updates the test to work with both C++17 and C++20 by
explicitly testing both modes and adding the extra expected notes when
in C++20 mode.