TestTP: Minor perf opt for ProvidedTypes.fs

[Thorium]

Description

• isNull is inline also in FSharp.Core. It's called in many places.
• Lazy load of ILAssemblyRefs: There is no point in spending so much time initially to construct ilAssemblyRefs which is only used by GetReferencedAssemblies() call
• Avoid looping 120 libraries through on most cases: Often convTypeRef is called to get some very basic base-type like a "System.Object" or "System.Int32" thus going through the dependency assemblies one-by-one can be a fall-back method rather than default implementation.

I would love to have some benchmark tests for ProvidedTypes.fs but the issue is that it's mostly used by FSharp.Compiler via a heavy IDE, and current unit-tests are not covering the majority of the process, so I don't have any idea how that could be done. Right now the best way seems to be to attach a VS to VS and open some type-provider and see what is going on.

[github-actions]

✅ No release notes required

[KevinRansom]

This file is a core part of the typeprovider SDK which is managed in: https://github.com/fsprojects/FSharp.TypeProviders.SDK

fsprojects/FSharp.TypeProviders.SDK#417

Is there some reason why it hasn't been merged there yet?

The file in this repo is a local fork. It would probably be best to make the change in it's real home, then port that file back to this repo, what do you think?

[Thorium]

I would expect it not to be merged because Don has more important things to focus on.
This PR does not change the external API at all.

Yes, I agree it could make sense not to maintain 2 copies of the file. Normally, I'd add the file as a Paket file reference, but Paket is not used here.

[Thorium]

The PR in TP SDK side merged now.