Disallow virtual modifiers

Modifiers are already a very complicated concept and we should not make it more difficult by allowing them to be virtual. Except for weird cases (adding another placeholder, putting it inside a loop, ...), you can get the same behaviour by adding a virtual function that is called from the modifier.

I would propose to remove this feature from the language with 0.9.0.

The issue proposes removing virtual modifiers from the Solidity language in the 0.9.0 release. Start by locating the compiler and language tests that cover virtual modifiers, then determine all behavior and compatibility cases that must be removed. Done means virtual modifiers are no longer accepted while ordinary modifier behavior and the proposed virtual-function alternative remain supported.

Written by the indexing model from the issue text.