[API Proposal] Make System.Reflection.Emit.ILGenerator constructor protected

[buyaa-n]

For new persisting AssemblyBuilder.Save implementation we have abstracted out all other Reflection.Emit public APIs with #78542, similar treatment needed for ILGenerator in order to save IL using MetadataBuilder. Literally all public methods of this type are virtual, but the constructor is internal, so I could not implement it outside of CoreLib

namespace System.Reflection.Emit
{
-   public partial class ILGenerator
+   public abstract class ILGenerator // there is no abstract method, probably make it abstract to avoid creating an instance. 
    {
-       internal ILGenerator() { }
+       protected ILGenerator() { }
        public virtual int ILOffset { get { throw null; } }
        public virtual void BeginCatchBlock(System.Type? exceptionType) { }
        public virtual void BeginExceptFilterBlock() { }
        // ... All public APIs a virtual except one, we can use the `byte arg` overload for this
        [System.CLSCompliantAttribute(false)]
        public void Emit(System.Reflection.Emit.OpCode opcode, sbyte arg) { }
    }
}

Contributes to #62956

CC @AaronRobinsonMSFT @jkotas @steveharter

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Issue Details

---

For new persisting AssemblyBuilder.Save implementation we have abstracted out all other Reflection.Emit public APIs with #78542, similar treatment needed for ILGenerator in order to save IL using MetadataBuilder. Literally all public methods of this type are virtual, but the constructor is internal, so I could not implement it outside of CoreLib

namespace System.Reflection.Emit
{
-   public partial class ILGenerator
+   public abstract class ILGenerator // there is no abstract method, probably make abstract to avoid creating an instance 
    {
-       internal ILGenerator() { }
+       protected ILGenerator() { }
        public virtual int ILOffset { get { throw null; } }
        public virtual void BeginCatchBlock(System.Type? exceptionType) { }
        public virtual void BeginExceptFilterBlock() { }
        // ... All public APIs a virtual except one, we can use the `byte arg` overload for this
        [System.CLSCompliantAttribute(false)]
        public void Emit(System.Reflection.Emit.OpCode opcode, sbyte arg) { }
    }
}

CC @AaronRobinsonMSFT @jkotas @steveharter

Author:	buyaa-n
Assignees:	-
Labels:	area-System.Reflection.Emit
Milestone:	-

[bartonjs]

Looks good as proposed.

The protected constructor and changing the type to abstract suggests that some of the virtual members should themselves flip to abstract (or that there's a protected abstract missing from the design). As a general reminder, abstract methods can't be added after the initial release where it becomes possible to actually extend the type, so for ILGenerator that means the current (.NET 8) release.

namespace System.Reflection.Emit
{
-   public partial class ILGenerator
+   public abstract partial class ILGenerator
    {
-       internal ILGenerator() { }
+       protected ILGenerator() { }
    }
}