[core] Add support for ConcurrentHashMap in AbstractAsKeyOfSetOrMap

[blemale]

Add support for ConcurrentHashMap in AbstractAsKeyOfSetOrMap, thus the error patterns ArrayAsKeyOfSetOrMap and ProtosAsKeyOfSetOrMap are available for ConcurrentHashMap.

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[blemale]

@googlebot I signed it!

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CLAs look good, thanks!

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[blemale]

@nglorioso could you take a look at this small PR?