Add SymbolDisplay option to append suffix for unannotated reference types

[cston]

There are currently SymbolDisplay options to append a suffix for the display of nullable and non-nullable reference types. Consider appending a suffix for unannotated types as well, perhaps ~.

string?
string!
string~

Perhaps rename the existing IncludeNonNullableTypeModifier option and use a single option to specify whether a suffix is added for non-nullable and unannotated types.

[cston]

cc @jcouv

[jcouv]

@jasonmalinowski @333fred @cston Do you think we need such an option?
I'm tempted to close this issue until we have an explicit driving scenario.

[333fred]

I would agree. No one but the compiler team would even understand what a ~ even means.