Introduce CallConvSwift and special register types to represent the Swift ABI calling convention

[kotlarmilos]

Description

Based on the design doc, this PR introduces types to represent each of the special registers.

The Swift programming language has a different ABI, runtime environment, and object model. According to the calling convention, the self context has dedicated registers, and it is always passed through them since it's heavily used. Methods calling other methods on the same object can share the self context. Errors are also handled through registers, so the caller needs to check for errors and throw if necessary. Implementing P/Invoke thunks should simplify registers juggling by using predefined set of registers in these scenarios.

The Mono runtime currently does not compile native wrappers with the LLVM backend. However, support for LLVM is essential for handling scenarios involving UnmanagedCallersOnly, where Swift code calls into C#. The implementation of LLVM support will be addressed in a subsequent PRs.

We need approval for the #64215 before merging this PR.

Fixes #64215

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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.

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Issue Details

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Description

Based on the design doc, this PR introduces types to represent each of the special registers.

The Swift programming language has a different ABI, runtime environment, and object model. According to the calling convention, the self context has dedicated registers, and it is always passed through them since it's heavily used. Methods calling other methods on the same object can share the self context. Errors are also handled through registers, so the caller needs to check for errors and throw if necessary. Implementing P/Invoke thunks should simplify registers juggling by using predefined set of registers in these scenarios.

We need approval for the #64215 before merging this PR.

Author:	kotlarmilos
Assignees:	kotlarmilos
Labels:	api-suggestion, area-System.Runtime.InteropServices
Milestone:	9.0.0

[kotlarmilos]

@AaronRobinsonMSFT please sign off on this from the Interop side.

[AaronRobinsonMSFT]

@kotlarmilos Note that unless @jkoritzinsky specifically asks for my sign-off, his review is sufficient for any interop related PRs. I do appreciate your dilligence in getting all the stakeholders involved though. Thanks!