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Issue Description:

The current Swift Standard Library implementations of JSONDecoder and PropertyListDecoder do not properly propagate `codingPath` for nested containers.

The attached playground shows the asymmetry between encoders and decoders (I created the playground to ensure that nested containers should have access to the full encoding path: encoders seem to, decoders don't – one or other should be true for both – I wanted to make sure it was not my misunderstanding.)

Using https://github.com/apple/swift/blob/master/stdlib/public/SDK/Foundation/JSONEncoder.swift

as a reference, the root cause of the problem can be found at lines: 1247, 1265, 1587, 1613; if nested containers use the decoder's codingPath and not their own to create children, well, the path will not be passed on.

Looking at the tests it seems there were not tests to examine decoder paths.