Emit StateMachineMethod and StateMachineHoistedLocalScopes into portable PDB format

[dsyme]

Roslyn has a capability to emit special debug information for C#/VB async methods.

F# doesn't emit this information. This means we must, in some way, be missing out on a debug capability supported by .NET debugging.

• clarify the user-facing experience this supports and verify it is not supported by F#
• collect the information in the compiler as we generate resumable code
• implement the code to emit this information
• see if it is useful beyond F# tasks.

Portable PDB spec is here: https://github.com/dotnet/runtime/blob/main/docs/design/specs/PortablePdb-Metadata.md

Also aplies to "StateMachine Local Scopes"
https://github.com/dotnet/roslyn/blob/3d8acbd585a75e9cdb3509623bf6c39ceb46b2c2/src/Compilers/Core/Portable/PEWriter/MetadataWriter.PortablePdb.cs

Here's an example of what they emit: https://github.com/dotnet/roslyn/blob/3d8acbd585a75e9cdb3509623bf6c39ceb46b2c2/src/Compilers/Core/Portable/PEWriter/MetadataWriter.PortablePdb.cs

       private void SerializeAsyncMethodSteppingInfo(AsyncMoveNextBodyDebugInfo asyncInfo, MethodDefinitionHandle moveNextMethod, int aggregateMethodDefRid)
        {
            Debug.Assert(asyncInfo.ResumeOffsets.Length == asyncInfo.YieldOffsets.Length);
            Debug.Assert(asyncInfo.CatchHandlerOffset >= -1);

            var writer = new BlobBuilder();

            writer.WriteUInt32((uint)((long)asyncInfo.CatchHandlerOffset + 1));

            for (int i = 0; i < asyncInfo.ResumeOffsets.Length; i++)
            {
                writer.WriteUInt32((uint)asyncInfo.YieldOffsets[i]);
                writer.WriteUInt32((uint)asyncInfo.ResumeOffsets[i]);
                writer.WriteCompressedInteger(aggregateMethodDefRid);
            }

            _debugMetadataOpt.AddCustomDebugInformation(
                parent: moveNextMethod,
                kind: _debugMetadataOpt.GetOrAddGuid(PortableCustomDebugInfoKinds.AsyncMethodSteppingInformationBlob),
                value: _debugMetadataOpt.GetOrAddBlob(writer));
        }

[dsyme]

My understanding is that, among other things, this means there will not be a false double-stepping point when moving into a function that returns task { ... } , becaus it makes the debugging step directly into the MoveNext