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.
Alexey Tsvetkov
87558873b5