Always parse macro expansions, regardless of language mode, and
eliminate the fallback path for very, very, very old object literals
like `#Color`. Instead, check for the feature flag for macro
declaration and at macro expansion time, since this is a semantic
restriction.
While here, refactor things so the vast majority of the macro-handling
logic still applies even if the Swift Swift parser is disabled. Only
attempts to expand the macro will fail. This allows us to enable the
macro-diagnostics test everywhere.
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.
