Fix dependent conformance mangling for multiple opaque result types.

We were always mangling based on the last generic parameter, which
makes sense when there is only one opaque result type, but is
incorrect when there are multiple opaque result types, e.g., due to
named or structural opaque result types.

Author: DougGregor

lib/AST/ASTMangler.cpp

@@ -3315,10 +3315,10 @@ void ASTMangler::appendAnyProtocolConformance(
     appendDependentProtocolConformance(path, genericSig);
   } else if (auto opaqueType = conformingType->getAs<OpaqueTypeArchetypeType>()) {
     GenericSignature opaqueSignature = opaqueType->getBoundSignature();
-    GenericTypeParamType *opaqueTypeParam = opaqueSignature.getGenericParams().back();
     ConformanceAccessPath conformanceAccessPath =
-        opaqueSignature->getConformanceAccessPath(opaqueTypeParam,
-                                                  conformance.getAbstract());
+        opaqueSignature->getConformanceAccessPath(
+          opaqueType->getInterfaceType(),
+          conformance.getAbstract());
 
     // Append the conformance access path with the signature of the opaque type.
     appendDependentProtocolConformance(conformanceAccessPath, opaqueSignature);