* Cache JS inferred class type symbol
Note that many sources merge into a single target, so the *source*
[links] is the one that caches the merged target.
The reason this is a problem is not that many sources merge into a
single target, but that both getTypeOfSymbol and getDeclaredTypeOfSymbol
end up calling mergeJSSymbols with the same [source,target] pair. The
merge should not happen twice.
* Remove more verbose debug assertion message
* Fix isJSConstructor check + update baselines
* inferClassSymbol cache now track multiple targets
