Drop net8.0 and modernize DllImportResolver invocation-list enumeration

[tannergooding]

Drops the out-of-support net8.0 target framework and modernizes the DllImportResolver invocation-list iteration to be allocation-free and AOT/trim-friendly.

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Drop net8.0 target framework

net8.0 is out of support, so it is dropped from the three remaining multi-targeting projects (ClangSharp, ClangSharp.Interop, and ClangSharp.UnitTests), leaving net10.0 as the sole target. The generator, CLI, and golden-test projects were already net10.0-only.

The now-dead #if NET8_0 branches in TranslationUnit (object vs System.Threading.Lock) and FieldDecl (string vs char StartsWith/EndsWith overloads) are collapsed to their net9.0+ forms, and the net8.0 reference in the copilot-instructions doc is updated.

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Use Delegate.EnumerateInvocationList to enumerate DllImportResolver

TryResolveLibrary previously iterated the multicast delegate's invocation list via System.Linq's GetInvocationList().Cast<DllImportResolver>(), which allocates a Delegate[] plus a LINQ enumerator and pulls in a LINQ dependency that is an AOT/trim smell. With net8.0 dropped, net9.0+ is guaranteed, so this now uses Delegate.EnumerateInvocationList(resolveLibrary), which returns an allocation-free struct enumerator.

Behavior is unchanged: same resolver order, first resolver returning a non-zero IntPtr wins and returns immediately, same fall-through when none resolve, and the existing resolveLibrary is not null guard is preserved. This was the only System.Linq use in the file, so the now-unused using System.Linq; is removed.

-            var resolvers = resolveLibrary.GetInvocationList().Cast<DllImportResolver>();
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-            foreach (DllImportResolver resolver in resolvers)
+            foreach (DllImportResolver resolver in Delegate.EnumerateInvocationList(resolveLibrary))

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Full solution builds with 0 warnings / 0 errors (net10.0 only) and all 3708 generator unit tests pass.

Note

This PR body was drafted by Copilot on my behalf.