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.
These have historically been defined as protocols in Objective-C
(under a pile of macros), but when imported into Swift they're classes
instead. Reverse this bit of magic by hard-coding the prefix "OS_" and
the header <os/object.h>, and emitting the classic 'foo_bar_t'-style
type names.
rdar://problem/29790636
