Add public APIs for persisted AssemblyBulder

[buyaa-n]

• Add public APIs for persisted AssemblyBulder, approve API shape:

namespace System.Reflection.Emit;

public partial class AssemblyBuilder
{
    // New API - note that it does not have RequiresDynamicCode annotation
    public static AssemblyBuilder DefinePersistedAssembly(AssemblyName name, Assembly coreAssembly, IEnumerable<CustomAttributeBuilder>? assemblyAttributes = null);

    public void Save(Stream stream);
    public void Save(string assemblyFileName);
    protected abstract void SaveCore(Stream stream);
}

• Use the public APIs for testing
• Refactor tests to make sure temp files disposed after test

Contributes to #92975

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-reflection-emit
See info in area-owners.md if you want to be subscribed.

Issue Details

---

• Add public APIs for persisted AssemblyBulder, approve API shape:

namespace System.Reflection.Emit;

public partial class AssemblyBuilder
{
    // New API - note that it does not have RequiresDynamicCode annotation
    public static AssemblyBuilder DefinePersistedAssembly(AssemblyName name, Assembly coreAssembly, IEnumerable<CustomAttributeBuilder>? assemblyAttributes = null);

    public void Save(Stream stream);
    public void Save(string assemblyFileName);
    protected abstract void SaveCore(Stream stream);
}

• Use the public APIs for testing
• Refactor tests to make sure temp files disposed after test

Author:	buyaa-n
Assignees:	-
Labels:	area-System.Reflection.Emit, new-api-needs-documentation
Milestone:	-

[stephentoub]

Was it not possible for someone externally to derive from AssemblyBuilder in .NET 8 and earlier? Adding a new abstract method will break anyone who did.

[buyaa-n]

We made the AssemblyBuilder abstract in .NET 8, before it was sealed. So only .NET 8 can be considered, and I think it's unlikely somebody would have derived from AssemblyBuilder and build their own implementation.

[stephentoub]

If it was possible to derive from AssemblyBuilder in .NET 8, then this is a binary breaking change. I think we need to make it virtual with the base implementation throwing.
cc: @terrajobst

[buyaa-n]

Done

[steveharter]

LGTM; some misc comments. Thanks for this important feature!

[steveharter]

nit: revert this file

[buyaa-n]

it is reverted, could not revert the line ending

[buyaa-n]

Failures unrelated and known