Flush diagnostics before destroying Clang. If we see an `inline Cpp` and
`Cpp` initialization failed, recover by skipping the inline code rather
than CHECK-failing.
For #6830, add support for inline C++ fragments as a declaration rather
than as a packaging directive. For now, this uses `inline Cpp
<string-literal>;` as syntax. The prior `import Cpp inline
<string-literal>;` is left alone for the time being. We can decide
separately whether to remove that.
`inline Cpp` requires that there was at least one `import Cpp`. It's not
clear to me if that's the right design long-term, but it seems
reasonable for now.
Assisted-by: Gemini via Google Antigravity
Richard Smith