Private functions not overridden by other contracts

[davidBar-On]

Fixes #11889

Suggested fix for preventing error about overriding private functions by inheriting/other contracts. The fix is based on the previous suggested fix by PR #13328, with the enhancements required by @ekpyron in that PR to make sure during code generation that private functions are not used by other contracts. For that, a new origCallingContract item was added to FunctionCallAnnotation, that points to the contract that calls the function. This field is used to verify that a private function is called by the defining contract, instead of using mostDerivedContract, since in case of a contract calling internal library function that calls a library's private function, mostDerivedContract points to the contract and not the library.

The test cases are practically the same as suggested by in PR #13328.

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[ekpyron]

I apologize for taking our time with getting back to you on this - the test coverage now indeed looks more reassuring.
It also has codegen assertions now as a safety net.
This will still need very careful review, but I think we can work towards merging it. It will definitely need a Changelog entry to start with.