[llvm] [PassInstrumentation] Make getPassNameForClassName return optional (PR #150029)

[Nikita Popov via llvm-commits]

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@@ -33,7 +34,10 @@ PassInstrumentationCallbacks::getPassNameForClassName(StringRef ClassName) {
       Fn();
     ClassToPassNameCallbacks.clear();
   }
-  return ClassToPassName[ClassName];
+  auto PassNameIter = ClassToPassName.find(ClassName);
+  if (PassNameIter != ClassToPassName.end())
+    return PassNameIter->second;
+  return {};
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nikic wrote:

```suggestion
  return "";
```
is a bit clearer, I think.

https://github.com/llvm/llvm-project/pull/150029