Add a warning against unnecessary use of "!"

[rrousselGit]

When using non-nullable types, it is possible to write the following:

int? a;
int? b = a!;

While this is valid code, it doesn't make sense to do a! when a simple a would be enough.
Since b is nullable, there is no reason to assert that a is not null, as any usage of b would handle the null case.

This is problematic because as we refactor code, it is possible for b to change from non-nullable to nullable.
So while the ! used to make sense, after refactoring it is instead harmful.

This should also support function calls:

void function(int? b) {}

function(a!); // do function(a) instead

[rrousselGit]

closing as this is handled my unnecessary_null_checks