[Swiftify] Add return pointer support (#78571)
* Import __counted_by for function return values
Instead of simply passing a parameter index to _SwiftifyInfo, the
_SwiftifyExpr enum is introduced. It currently has two cases:
- .param(index: Int), corresponding to the previous parameter index
- .return, corresponding to the function's return value.
ClangImporter is also updated to pass this new information along to
_SwiftifyImport, allowing overloads with buffer pointer return types to
be generated. The swiftified return values currently return Span when
the return value is marked as nonescaping, despite this not being sound.
This is a bug that will be fixed in the next commit, as the issue is
greater than just for return values.
* Fix Span variant selection
There was an assumption that all converted pointers were either
converted to Span-family pointers, or UnsafeBufferPointer-family
pointers. This was not consistently handled, resulting in violating the
`assert(nonescaping)` assert when the two were mixed. This patch removes
the Variant struct, and instead each swiftified pointer separately
tracks whether it should map to Span or UnsafeBufferPointer.
This also fixes return pointers being incorrectly mapped to Span when
marked as nonescaping.
