[ModuleInterface] Update wording of the "failed to build module" error

[xymus]

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.

[beccadax]

This is an improvement, but I wonder if we could go further. For instance:

• Would people be more likely to look at the specific failures if they came after this message instead of before it?
• Would it be better to emit this diagnostic as a note attached to each error diagnosed from the module interface? (We'd also need a fallback in case building somehow failed without emitting any errors.)
• Would it make sense to split some parts of this diagnostic, like the various hypothetical explanations, into separate notes attached to the top-level error?

[xymus]

@beccadax I'm hesitant on how to reach a good middle ground that would display well in both Xcode and the build logs. A notable problem with these diagnostics in the build logs are the prefix that has the full path to the swiftinterface files in the SDK. Moving the hypothetical explanations to notes would work well for direct imports for example but would become more verbose when displayed for an indirect import.

[xymus]

@swift-ci Please smoke test

[xymus]

@swift-ci Please smoke test