Make @Builder work with both Default and throwing constructor

[emlun]

Currently, if the constructor throws a checked exception then @Builder must be on the constructor in order for the checked exception to propagate to the throws clause of the builder() method. However, @Builder.Default requires that the @Builder annotation be set on the class. It is possible to annotate both the class and the constructor, but then the constructor one gets skipped. This means it is currently impossible to use @Builder.Default simultaneously with a throwing constructor (except perhaps by manually implementing the builder() method), as demonstrated by the tests added in commit d905be5.

This attempts to fix that. If you set @Builder on both the class and the constructor, the constructor's checked exceptions will now be added to the builder() method initially generated by the class annotation. I have also verified that this works in the real-world context where I encountered the issue.

[emlun]

Oh, and I'm basing the branch on v1.18.20 because I couldn't get the tests to pass on any later commits. I hope that doesn't cause any trouble, I'm happy to fix the merge conflicts once that becomes relevant.