We already have something called "module interfaces" -- it's the
generated interface view that you can see in Xcode, the interface
that's meant for developers using a library. Of course, that's also a
textual format. To reduce confusion, rename the new module stability
feature to "parseable [module] interfaces".
Textual module interfaces don't actually depend on SILGen, so we
shouldn't need to run SILGen (or serialize an entire binary module) if
we're just trying to emit a textual interface. On the other hand, if
we /are/ going to run SILGen and then SIL diagnostics, we shouldn't
delay those diagnostics by spending time emitting a textual interface,
or for that matter a TBD file.
Using this, update all the ModuleInterface tests that use
`-emit-module -o /dev/null` to use `-typecheck` instead, except for
those using `-merge-modules`.
The special 'Builtin' module is implicitly imported via -parse-stdlib,
and can't be found via normal import resolution, so we shouldn't print
it in a textual interface.
