Migration tool for Enum to string attempts to alter column on every migration

[ctrlaltdan]

Issue with migrations tool attempting to alter string enum property on every add-migration step when using HasConversion<string>().

Steps to reproduce

• Create an entity with an enum property
• Add the .HasConversion<string>() to the model builder
• Add a migration (works fine)
• Add an additional migration (tool attempts to alter already correct property)

Repro

https://github.com/dubs999/efcore-string-enum-bug

Further technical details

EF Core version: 2.1.0-preview-1
Database Provider: (e.g. Microsoft.EntityFrameworkCore.SqlServer)
Operating system:
IDE: (e.g. Visual Studio 2017 15.4)

[bricelam]

Related to #11297. Snapshot should contain .IsRequired() on the converted property.