[WIP]: MethodImplOption.Async support

[TheAngryByrd]

Description

Fixes #19056

Checklist

• Test cases added

• Performance benchmarks added in case of performance changes

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[github-actions]

❗ Release notes required

@TheAngryByrd,

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Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.0.md	No release notes found or release notes format is not correct

[T-Gro]

I assume this will fail when targeting desktop framework, you might need to use TheoryForNETCOREAPPAttribute in the test declaration.

[majocha]

We should make sure ildasm version that the CI uses actually does support async flag

[TheAngryByrd]

We should make sure ildasm version that the CI uses actually does support async flag

Yeah, I had that changed locally to use the latest but I didn't commit it. Definitely part of my verification work.