ILLink handling of `Type.GetType` doesn't resolve types from corelib

[sbomer]

Repro:

using System;
var t = Type.GetType("G`1[[System.Int32]]");
Console.WriteLine(t.ToString());

class G<T> {}

If this app is published as PublishTrimmed, the app nullrefs on t.ToString() because ILLink's intrinsic handling for type names did not resolve System.Int32 and as a result didn't mark the type G.

[dotnet-policy-service]

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