[LibraryImportGenerator] Use basic forwarder in down-level support if any parameters can't be marshalled

[elinor-fung]

Alternative to #103973 (and #104246)

In down-level support, when determining marshalling generators for types, we fall back to just forwarding the parameter without erroring. This means that we end up generating a stub that marshal some parameters and just silently forwards others when all of these are true:

• target framework is down-level (below .NET 7)
• the signature is not blittable (parameters require marshalling and/or SetLastError=true)
• no parameters are identified via an ITypeBasedMarshallingInfoProvider as missing marshalling support
• at least one parameter cannot be marshalled and falls back to just forwarding the parameter

This is not a problem in non-down-level scenarios, as we error if we can't marshal any parameters. This is also not a problem if any parameters are identified as unsupported via an ITypeBasedMarshallingInfoProvider, as we do correctly check that case and simply forward to DllImport.

This change makes it so that in down-level scenarios, if any type falls back to forwarding, we print a basic DllImport instead of a stub with mixed marshalling and forwarding.

We have existing tests for validating that any unsupported type resulted in a forwarder even when other types were supported:

runtime/src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/Compiles.cs

Lines 524 to 527 in 69f7006

	// Confirm that if support is missing for any type (like arrays), we fall back to a forwarder even if other types are supported.
	{
	string code = CodeSnippets.BasicReturnAndParameterByValue("System.Runtime.InteropServices.SafeHandle", "int[]", CodeSnippets.LibraryImportAttributeDeclaration);
	yield return new object[] { ID(), code, TestTargetFramework.Net6, true };

However, they only used types that were explicitly flagged as not supported by an ITypeBasedMarshallingInfoProvider that handled the type. They were also using SafeHandle as the non-blittable type that was supported down-level, but that is no longer the case in main (has been removed in .NET 9 so that we only supporting marshalling via SafeHandleMarshaller).

[elinor-fung]

/azp run runtime-libraries-coreclr outerloop

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