[IRGen] Fix witness table miscompilation.

If a foreign type conforms to a protocol, and that conformance is not
resilient, we were emitting an instantantiation function but it would
never be called because we weren't emitting the `GenericWitnessTable`
that would have contained a reference to it.

This was happening because of a missing `isSynthesizedNonUnique()` call
in `isDependentConformance()`.

rdar://97290618

Author: al45tair

lib/IRGen/GenProto.cpp

@@ -961,11 +961,13 @@ static bool isDependentConformance(
     if (assocConformance.isInvalid())
       return false;
 
+    auto rootConformance = assocConformance.getConcrete()->getRootConformance();
+
+    // [*] This condition must be true if getConformanceInfo() would return
+    //     an AccessorConformanceInfo().
     if (assocConformance.isAbstract() ||
-        isDependentConformance(IGM,
-                               assocConformance.getConcrete()
-                                 ->getRootConformance(),
-                               visited))
+        isSynthesizedNonUnique(rootConformance) ||
+        isDependentConformance(IGM, rootConformance, visited))
       return true;
   }
 
@@ -2142,6 +2144,9 @@ IRGenModule::getConformanceInfo(const ProtocolDecl *protocol,
   // so in theory we could allocate them on a BumpPtrAllocator. But there's not
   // a good one for us to use. (The ASTContext's outlives the IRGenModule in
   // batch mode.)
+  //
+  // N.B. If you change this condition, you may need to update the condition
+  //      marked [*] in isDependentConformance().
   if (isDependentConformance(rootConformance) ||
       // Foreign types need to go through the accessor to unique the witness
       // table.