[clang] [APINotes] Add SWIFT_RETURNS_(UN)RETAINED support (PR #118938)

[Saleem Abdulrasool via cfe-commits]

================
@@ -373,6 +373,13 @@ void ReadFunctionInfo(const uint8_t *&Data, FunctionInfo &Info) {
       endian::readNext<uint16_t, llvm::endianness::little>(Data);
   Info.ResultType = std::string(Data, Data + ResultTypeLen);
   Data += ResultTypeLen;
+
+  unsigned SwiftReturnOwnershipLength =
+      endian::readNext<uint16_t, llvm::endianness::little>(Data);
+  Info.SwiftReturnOwnership = std::string(reinterpret_cast<const char *>(Data),
+                                          reinterpret_cast<const char *>(Data) +
+                                              SwiftReturnOwnershipLength);
----------------
compnerd wrote:

What do you think of making this stronger and using a `StringSwitch` to encode this as an enumerated type? That would compress the encoded serialisation, ensure that we catch an errant representation allowing better diagnostics, and just make this generally less confusing to see with the string representation.

https://github.com/llvm/llvm-project/pull/118938