Update GeneratedDllImportAttribute CharSet -> StringMarshalling

[elinor-fung]

• Update GeneratedDllImportAttribute CharSet to StringMarshalling
• Update usage in runtime libraries
• Make fixer handle CharSet.Unicode -> StringMarshalling.Utf16
• Did not make fixer handle CharSet.Ansi or CharSet.Auto
  • For ANSI, we could look through all parameters and the return and mark them MarshalAs as needed, but I figured we would just make the fixer handle this once we propose and do some AnsiStringMarshalling using the design in Attributes and Analyzer Diagnostics to support custom struct marshalling in the DllImport Source Generator #46838 and can use that for StringMarshallingCustomType - would be more in line with the original code's intent
  • For Auto, we could define two different p/invokes and update all the callsites to conditionally call different ones. Honestly not convinced this is worth it - we had one use case in all of runtime and that was for invoking into hostpolicy.
• StringMarshalling enum is just added internally (like GeneratedDllImportAttribute) for now. Once we update/rename GeneratedDllImportAttribute per the approved API, we can expose them.

I will rebase / update after #65343 goes in. (done)

Contributes to #46822. #61326

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Issue Details

---

• Update GeneratedDllImportAttribute CharSet to StringMarshalling
• Update usage in runtime libraries
• Make fixer handle CharSet.Unicode -> StringMarshalling.Utf16
• Did not make fixer handle CharSet.Ansi
  • We could look through all parameters and the return and mark them MarshalAs as needed, but I figured we would just make the fixer handle this once we propose and do some AnsiStringMarshalling using the design in Attributes and Analyzer Diagnostics to support custom struct marshalling in the DllImport Source Generator #46838 and can use that for StringMarshallingCustomType - would be more in line with the original code's intent
• StringMarshalling enum is just added internally (like GeneratedDllImportAttribute) for now. Once we update/rename GeneratedDllImportAttribute per the approved API, we can expose them.

I will rebase / update after #65343 goes in.

Contributes to #46822. #61326

Author:	elinor-fung
Assignees:	elinor-fung
Labels:	area-System.Runtime.InteropServices
Milestone:	-

[elinor-fung]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[elinor-fung]

The outerloop failures are

• System.Runtime.Tests Outerloop "killed" on Debian.10 and Debian.11 #58616
• busy hang running System.Text.RegularExpressions.Tests.RegexMatchTests.StressTestDeepNestingOfLoops #65271

This should be ready to review.