[C++20] [Modules] Diagnose if import statement lakcs a semicolon
Close https://github.com/llvm/llvm-project/issues/121066
Now we will diagnose that the import statement lacks a semicolon as
expected. Note that the original "not found" diagnose message remains.
I meant to remove that, but the test shows it might be more complex
process (other unexpected diagnose shows up). Given the importance of
the issue, I chose to not dig deeper.