[ABI] [Mangling] Disambiguate protocol-conformance-ref better

[jrose-apple]

Fix to #21397. The mangling operator "HP" has to distinguish between "protocol" and "protocol + module", not between the presence or absence of protocol-conformance-ref. New grammar:

protocol-conformance-ref ::= protocol
protocol-conformance-ref ::= protocol module 'HP'

rdar://problem/46735592, again. This is a narrow fix for that issue; the problem only occurs when mangling a reference to a non-retroactive conformance, but it's happening in a context where we only need retroactive conformances anyway. I'll look into that next and if it's simple enough maybe we can take that too.

[jrose-apple]

@swift-ci Please test

[jrose-apple]

@swift-ci Please test source compatibility

[rjmccall]

This is definitely a better mangling.

So something trying to identify what module a conformance belongs to would need to figure out what the module dependency relationship is between the type and the protocol, right? If we wanted to find a conformance dynamically, we'd probably have to check in both places. Seems a little unfortunate.

[jrose-apple]

That's true. If we think that's important to encode, we could always include the module name if it's different from the type xor the protocol, or use a different operator, or use a dummy module name to indicate "same as type" xor "same as protocol" (with the absence of any module name indicating the other).

[rjmccall]

I think you mean "either the type or the protocol", right? I think I'd be happier with a couple of operators for each of those two cases, yeah, so that it would be:

  protocol-conformance-ref ::= protocol '??'       // conformance from protocol's module
  protocol-conformance-ref ::= protocol '??'       // conformance from type's module
  protocol-conformance-ref ::= protocol module 'HP'    // retroactive conformance

...although all of the manglings here are pretty weird and worrisome, so maybe I should just put my head back in the sand.

[jrose-apple]

Continuing in #21450, since @DougGregor approved the secondary change there.