me: fix regression in enum column type diffing

[tomhoule]

This regression was reported by users in
prisma/prisma#16180

It was introduced on b6400f1 by the
same change that caused the other regression that was fixed in
eb39236.

Symptoms: enum migrations became non-idempotent in many cases, depending
on the ordering of the migrations in the Prisma schema.

In that refactoring, we started storing enum ids instead of enum names
in ColumnTypeFamily::Enum(_). That makes the ColumntypeFamily type
smaller, easier to deal with with regards to ownership (EnumId is
Copy), and ready for multi-schema, because the name can be ambiguous
and does not carry schema information, whereas the EnumId does. But
more relevant to this commit: it makes comparing two
ColumnTypeFamily inaccurate when the types come from two different
schemas, like in sql_schema_differ.

That in turn changed the signature of column_type_family_as_enum(),
leading to the differ comparing enum ids instead of enum names, which
directly causes the false positives in diffing, because the same enum
has a different ID in each of the two schemas. The code that caused the
regression does not appear in b6400f1,
it uses code that was changed there.

This is also a problem with the generated implementation of PartialEq
for ColumnTypeFamily. Since it would be a larger refactoring, we only
deal with this where relevant (sql_schema_differ) and leave removing the
dangerous PartialEq implementation to another PR. This is the issue:
#3373.

How could this have been caught?

• Tests that check that migrations are idempotent with multiple enums,
  all used in model columns not defined in alphabetical order (sad but true).
  • This chimes with the theme of us not testing with large enough
    Prisma Schemas.
• migrations-ci could have caught this. We are neglecting it at the
  moment.

[eviefp]

👍