Fix #3702: Don't rewrite GetValueOrDefault() to == true in ?. chains

Turning bool?.GetValueOrDefault() into `== true` ends an enclosing
null-conditional chain, so `c?.Flag.GetValueOrDefault()` became
`(c?.Flag == true)` of type bool instead of bool?. A following `?? false`
then fails to compile (CS0019), and a following `.ToString()` yields
"False" instead of null when c is null. Keep the call when its target is
part of a ?. chain.

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs

@@ -380,7 +380,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		}
 
-		bool IsNullConditional(Expression target) => target switch {
+		internal static bool IsNullConditional(Expression target) => target switch {
 			UnaryOperatorExpression { Operator: UnaryOperatorType.NullConditional } => true,
 			MemberReferenceExpression member => IsNullConditional(member.Target),
 			InvocationExpression invocation => IsNullConditional(invocation.Target),