[clang] [clang][ExtractAPI] Fix assertion on invalid UTF-8 in doc comments (PR #212394)

[Prajwal Nadig via cfe-commits]

================
@@ -0,0 +1,16 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -extract-api --pretty-sgf --emit-sgf-symbol-labels-for-testing \
----------------
snprajwal wrote:

Nit: we're not using test labels here, so `--emit-sgf-symbol-labels-for-testing` is not useful. We can remove it. It would also be nice to verify that no diagnostics are emitted:


```suggestion
// RUN: %clang_cc1 -extract-api --pretty-sgf --verify \
```

https://github.com/llvm/llvm-project/pull/212394