Fix handling of polyfill'd internal UnmanagedCallersOnlyAttribute leaking from a referenced assembly

Author: AArnott

src/Microsoft.Windows.CsWin32/Generator.cs

@@ -106,7 +106,7 @@ public Generator(string metadataLibraryPath, Docs? docs, GeneratorOptions option
         this.canUseUnsafeAdd = this.compilation?.GetTypeByMetadataName(typeof(Unsafe).FullName)?.GetMembers("AsRef").Any() is true;
         this.canUseUnsafeNullRef = this.compilation?.GetTypeByMetadataName(typeof(Unsafe).FullName)?.GetMembers("NullRef").Any() is true;
         this.canUseUnsafeSkipInit = this.compilation?.GetTypeByMetadataName(typeof(Unsafe).FullName)?.GetMembers("SkipInit").Any() is true;
-        this.canUseUnmanagedCallersOnlyAttribute = this.compilation?.GetTypeByMetadataName("System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute") is not null;
+        this.canUseUnmanagedCallersOnlyAttribute = this.FindTypeSymbolsIfAlreadyAvailable("System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute").Count > 0;
         this.canUseSetLastPInvokeError = this.compilation?.GetTypeByMetadataName("System.Runtime.InteropServices.Marshal")?.GetMembers("GetLastSystemError").IsEmpty is false;
         this.unscopedRefAttributePredefined = this.FindTypeSymbolIfAlreadyAvailable("System.Diagnostics.CodeAnalysis.UnscopedRefAttribute") is not null;
         this.runtimeFeatureClass = (INamedTypeSymbol?)this.FindTypeSymbolIfAlreadyAvailable("System.Runtime.CompilerServices.RuntimeFeature");