[clang] [clang] Make serialized diagnostics more reliable (PR #100681)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

Could we simplify this even further by removing `DiagnosticConsumer::finish()` entirely, moving the code in `SDiagsWriter::finish()` to `SDiagsWriter::~SDiagsWriter()` and ensuring the **destructor** gets called even with `-disable-free`?

Also, can you please create and link a downstream PR that adds a test case?

https://github.com/llvm/llvm-project/pull/100681