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.
The code already forward declared strutcs and enums. This patch extends
the logic to also forward declare classes. Unfortunately, there was some
fallout because some traits ended up defined multiple times for some
classes, so the code is now extended to only conditionally emit these
traits if no forward declaration was emitted for the type yet.
rdar://124022242
