Disable `-Worphans` by default in non-`library` targets?

[geekosaur]

Describe the feature request
The notion of "orphan instance" doesn't apply to anything but libraries, so executables, test suites, and benchmarks shouldn't check for them unless specifically enabled in ghc-options.

One might think that this would also apply to private libraries, but instances are global so orphans can still cause problems if someone else has defined the same (orphan) instance somewhere.

Additional context
This came up on Matrix #haskell.