[clang] [Clang] Fix assertion failure in SDiagsWriter when finish() is not called (PR #181712)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 86219 tests passed
* 1262 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### Clang
<details>
<summary>Clang.Tooling/serialize-diagnostics.cpp</summary>

```
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 1
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang-check /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Tooling/serialize-diagnostics.cpp -- -Wunknown-warning-option --serialize-diagnostics /home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/test/Tooling/Output/serialize-diagnostics.cpp.tmp.dia 2>&1 | /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/FileCheck /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Tooling/serialize-diagnostics.cpp --allow-empty
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang-check /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Tooling/serialize-diagnostics.cpp -- -Wunknown-warning-option --serialize-diagnostics /home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/test/Tooling/Output/serialize-diagnostics.cpp.tmp.dia
# note: command had no output on stdout or stderr
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/FileCheck /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Tooling/serialize-diagnostics.cpp --allow-empty
# .---command stderr------------
# | error: no check strings found with prefix 'CHECK:'
# `-----------------------------
# error: command failed with exit status: 2

--

```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/181712