RFR 8221118: Avoid eagerly creating JCDiagnostic for CompletionFailures

[Maurizio Cimadamore]

On 28/03/2019 13:43, Ron Shapiro wrote:
> From what the code is currently doing, `() -> null` could be made into 
> just `null`. On my own, I didn't feel comfortable introducing that and 
> preferred to remain consistent, but if you think I should make the 
> change I defer to you.

Maybe we could have an override that takes no 'supplier', and instead 
uses a predefined () -> null supplier defined somewhere, so that it 
doesn't get created all the time.

Thanks for the additional data points.

Maurizio