Suppress cascaded diagnostics for unused vars and funcs declared improperly

A typical situation appears in this test:

        [Fact, WorkItem(10521, "https://github.com/dotnet/roslyn/issues/10521")]
        public void LabeledLocalFunctionInIf()
        {
            var source = @"
class Program
{
    static void Main(string[] args)
    {
        if () // typing at this point
a:      int Add(int x, int y) => x + y;
    }
}
";
            VerifyDiagnostics(source,
                // (6,13): error CS1525: Invalid expression term ')'
                //         if () // typing at this point
                Diagnostic(ErrorCode.ERR_InvalidExprTerm, ")").WithArguments(")").WithLocation(6, 13),
                // (7,1): error CS1023: Embedded statement cannot be a declaration or labeled statement
                // a:      int Add(int x, int y) => x + y;
                Diagnostic(ErrorCode.ERR_BadEmbeddedStmt, "a:      int Add(int x, int y) => x + y;").WithLocation(7, 1),
                // (7,1): warning CS0164: This label has not been referenced
                // a:      int Add(int x, int y) => x + y;
                Diagnostic(ErrorCode.WRN_UnreferencedLabel, "a").WithLocation(7, 1),
                // (7,13): warning CS0168: The variable 'Add' is declared but never used
                // a:      int Add(int x, int y) => x + y;
                Diagnostic(ErrorCode.WRN_UnreferencedVar, "Add").WithArguments("Add").WithLocation(7, 13)
                );
        }

The latter two diagnostics should be suppressed by the compiler.