PrintAsClang is supposed to emit declarations in the same order regardless of the compiler’s internal state, but we have repeatedly found that our current criteria are inadequate, resulting in non-functionality-affecting changes to generated header content. Add a diagnostic that’s emitted when this happens soliciting a bug report.
Since there *should* be no cases where the compiler fails to order declarations, this diagnostic is never actually emitted. Instead, we test this change by enabling `-verify` on nearly all PrintAsClang tests to make sure they are unaffected.
This did demonstrate a missing criterion that only mattered in C++ mode: extensions that varied only in their generic signature were not sorted stably. Add a sort criterion for this.
Previously (a03c40cb2c) we assumed all Swift enums were non-frozen in
ObjC, a weird choice in retrospect. Now that we actually distinguish
frozen and non-frozen enums in Swift, we can use the
'enum_extensibility' attribute to mark them as open or closed in ObjC.
Note that this only matters for Swift libraries compiled with
-enable-resilience, i.e. those that might get a new implementation at
runtime. Everyone else is now declaring a "closed" enum, matching the
behavior in Swift.
