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.
This fixes a compiler crash that happened when emitting a Clang header for a Swift module that declares multiple macros with the same base name and different argument names.
Swift macros are not currently designed to be exposed to C++. This teaches the compiler to explicitly mark them as unavailable in C++.
rdar://117969472 / resolves https://github.com/apple/swift/issues/69656
