Disallow visibility modifiers on constructors

## Abstract
Constructors can currently have a visibility modifier (`internal` if abstract, `public` otherwise), which has no real effect. This is redundant and should be disallowed.

## Motivation
In Solidity 0.6.x specifying visibility of a constructor was mandatory. It could be either `internal` or `public`. An internal constructor could not be "invoked" from the outside, making the contract undeployable on its own. Such constructor could only be invoked internally, from within a derived contract.

However, the concept did not map 1:1 to the way visibility works for ordinary functions and there were various opinions on how to adjust it (#4392). On the other hand, it lined up exactly with the notion of an abstract contract and was therefore redundant (#8162). In 0.7.0 (#8992) we decided to allow omitting the visibility and disallow abstract contracts with a public constructor as well as non-abstract ones with an internal constructor.

We left the possibility of using the abstract/internal and non-abstract/public combinations open (with a warning). It was argued that this is important for libraries to keep portability across different compiler versions (#9399). Still, the intention was to disallow those eventually as well and now enough time has passed that this should no longer be controversial.

## Specification

Report an error when any visibility specifier is used on a contract constructor.

## Backwards Compatibility

This is a breaking syntax change.

The issue names no files, tests, or entry points. Start by locating the compiler handling for constructor visibility and its diagnostic tests; done means every visibility specifier on a constructor produces an error while constructors without one remain accepted.

Written by the indexing model from the issue text.