Fix #3180: Spell out type arguments of a null-conditional extension call

When the extension method call cannot be written without type arguments
(another extension method with the same name and a different arity is
found first), the call falls back to the static form. For 'a?.M()' that
fallback produced 'Enumerable.Count(s?)', which is not valid C#. Writing
the type arguments excludes the extension methods of a different arity,
so for a null-conditional target the type arguments are tried before
giving up.

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -932,6 +932,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task Issue3180([ValueSource(nameof(roslynOnlyOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task AssemblyCustomAttributes([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
 		{