Fix source element for JsSwitch generated from 'when'

Before this change we set JsSwitch source to subject's JsExpression.
We ignore these types of source elements during:
1. js ast serialization (see 'JsAstSerializer.extractLocation');
2. source map generation (see 'SourceMapBuilderConsumer.addMapping').

However we might add indent during source generation
(see 'JsToStringGenerationVisitor.pushSourceInfo').

That seemingly broken 'testEnumEntryRemoved' in
'IncrementalJsCompilerRunnerTestGenerated.ClassHierarchyAffected',
because deserialized ast during incremental build would not
have source element at all (JsExpression was ignored), but ast
during rebuild would have JsExpression source element.

js/js.translator/src/org/jetbrains/kotlin/js/translate/expression/WhenTranslator.kt

@@ -163,7 +163,7 @@ private constructor(private val whenExpression: KtWhenExpression, context: Trans
                 lastEntry.statements += JsBreak().apply { source = entry }
                 members
             }
-            Pair(JsSwitch(subjectSupplier(), switchEntries).apply { source = expression }, nextIndex)
+            Pair(JsSwitch(subjectSupplier(), switchEntries).apply { source = whenExpression }, nextIndex)
         }
         else {
             null

js/js.translator/testData/lineNumbers/whenEntryWithMultipleConditions.kt

@@ -18,4 +18,4 @@ fun box(x: Int) {
     )
 }
 
-// LINES: 19 4 4 3 4 6 9 9 6 11 13 13 11 16 16 3 2
+// LINES: 19 4 4 3 3 4 6 9 9 6 11 13 13 11 16 16 3 2