Migrations: Support removing IDENTITY from a column

[MaximBalaganskiy]

When an SQL table is created with Id column it gets IDENTITY by default. Adding a ValueGeneratedNever method call later generates a migration which does not remove this clause.

Example repo. It has two migrations - Initial will generate a table with an Identity column. The second one was supposed to remove that Identity.

[bricelam]

Notes for triage: If I remember right, we're mimicking EF6 behavior because removing IDENTITY requires a rebuild.

[divega]

BTW, we said in the past that we wanted to give the user some kind of warning rather than silently ignoring changes that would cause table rebuilds.

[rowanmiller]

Dependent upon #329

[divega]

This seems to be a dupe since we have decided to warn for RTM (tracked in #4783) and we need table rebuilds to work for any further improvement (tracked in #329).

cc @rowanmiller

[bricelam]

Closing as a dupe of #5345. We throw now.