Introduce MutableDiagnosticsWithSuppression and use it in BindingTrace implementations

Exposes the fact that Diagnostics is in fact a mutable entity with readonly API
Update readonly view when diagnostics are modified
Allows to avoid recreating DiagnosticsWithSuppression every time getDiagnostics() is called in DelegatingBindingTrace
  which led to severe performance issues for large files with lots of diagnostics

compiler/tests/org/jetbrains/jet/JetTestUtils.java

@@ -124,7 +124,7 @@ public class JetTestUtils {
                 @NotNull
                 @Override
                 public Diagnostics getDiagnostics() {
-                    throw new UnsupportedOperationException(); // TODO
+                    return Diagnostics.EMPTY;
                 }
 
                 @Override