Improve SA1129 code fix for enum types

[sharwell]

Currently the SA1129 code fix replaces new TaskState() with default(TaskState). An improved code fix for enum types would be the following:

• If a single member Member in the enum declaration T has the value 0, replace new T() with T.Member
• Otherwise, replace new T() with default(T) (i.e. fall back to the current behavior)

[nbarbettini]

For this and #1973, does it makes sense to just keep expanding the existing code fix with additional branches? Or should this be split into separate diagnostics/fixes?

[vweijsters]

Either way you will have to create additional branches, as splitting into separate diagnostics/fixes will require exceptions to be added to the current implementation.

I would therefore prefer to go with extending the existing code fix, as adding exceptions for more specific diagnostics / fixes have not been problem free (For examples I'm thinking of SA1003 and SA1020-SA1024, but there are more).

[nbarbettini]

Grabbing this.

[nbarbettini]

@sharwell To clarify, do you mean "enum declaration T has only one member", or "enum declaration T has one member with value 0, among many members"?

[sharwell]

I meant the latter. 😄