[JS] Fix a compiler crash when generating sourcemaps

It is possible that the source info has the type `KtPureElement`.
For example, the serialization plugin may create a synthetic companion
object for a `@Serializable`-annotated class.
This results in an instance of
the `SyntheticClassOrObjectDescriptor.SyntheticDeclaration` class being
indirectly set as source info. This class implements the `KtPureElement`
interface.

js/js.sourcemap/src/org/jetbrains/kotlin/js/sourceMap/SourceMapBuilderConsumer.kt

@@ -8,6 +8,7 @@ import com.intellij.psi.PsiElement
 import org.jetbrains.kotlin.js.backend.SourceLocationConsumer
 import org.jetbrains.kotlin.js.backend.ast.JsLocationWithSource
 import org.jetbrains.kotlin.js.backend.ast.JsNode
+import org.jetbrains.kotlin.psi.KtPureElement
 import org.jetbrains.kotlin.resolve.calls.util.isFakePsiElement
 import org.jetbrains.kotlin.utils.addToStdlib.popLast
 import java.io.*
@@ -86,8 +87,8 @@ class SourceMapBuilderConsumer(
                     sourceInfo.startChar
                 )
             }
-            is JsNode -> { /* Can occur on legacy BE, not sure if it's a bug or not. */ }
-            else -> error("Unexpected sourceInfo: $sourceInfo")
+            is JsNode, is KtPureElement -> { /* Can occur on legacy BE */ }
+            else -> {}
         }
     }
 }