Consider banning string function

[pbiggar]

It's impossible to tell statically if the ToString function we are calling is one that we intend to call or merely the built-in Object.ToString(). This particularly came up for FQFnName.toString.

[pbiggar]

That wouldn't prevent string interpolation.

Look at https://www.nuget.org/packages/ToStringWithoutOverrideAnalyzer/

[StachuDotNet]

Folded into #5246