The error shown when the compiler fails to build a module from its
textual interface has been creating some confusion. This is a proposal
to make it more useful to the programmer not working in the SDK, insist
on looking at the previous errors first, put less emphasis on possible
compiler bugs, and always show alternatives to the incompatible
compilers issue as this is the most common case now.
Currently, when a swiftinterface file fails to load, we emit the specific diagnostics for the failures, followed by a generic “failed to load module ‘Foo’” message. This PR improves that final diagnostic, particularly when the cause may be that the interface was emitted by a newer compiler using backwards-incompatible syntax.
