a0d186073b
Make `FilteringDiagConsumer` also ignore -Wunused-result. Whether or not
such diagnostic is filtered depends on `CompilationOptions::IgnorePromptDiags`.
In particular, `IgnorePromptDiags` should _only_ be enabled for code parsed
via `Interpreter::EvaluateInternal()`. Thus, as of this commit `IgnorePromptDiags`
defaults to 0 in `makeDefaultCompilationOpts()`
The observable effect of this change is ignoring `-Wunused-result` for
wrapped code, e.g.
```c++
[[nodiscard]] int f() { return 0; }
// This yields `warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]`
void g() { f(); }
f(); // but this should not
```