[ABI] Use faux mangled names for associated conformances in witness tables

The current representation of an associated conformance in a witness
tables (e.g., Iterator: IteratorProtocol within a witness table for
Sequence) is a function that the client calls.

Replace this with something more like what we do for associated types:
an associated conformance is either a pointer to the witness table (once
it is known) or a pointer to a mangled name that describes that 
conformance. On first access, demangle the mangled name and replace the
entry with the resulting witness table. This will give us a more compact
representation of associated conformances, as well as always caching
them.

For now, the mangled name is a sham: it’s a mangled relative reference to
the existing witness table accessors, not a true mangled name. In time,
we’ll extend the support here to handle proper mangled names.

Part of rdar://problem/38038799.

lib/IRGen/GenMeta.cpp

@@ -788,15 +788,15 @@ namespace {
           continue;
 
         auto witness = entry.getAssociatedTypeProtocolWitness().Witness;
-        return getDefaultAssociatedConformanceAccessFunction(
-                 AssociatedConformance(Proto, association, requirement),
-                 witness);
+        AssociatedConformance conformance(Proto, association, requirement);
+        defineDefaultAssociatedConformanceAccessFunction(conformance, witness);
+        return IGM.getMangledAssociatedConformance(nullptr, conformance);
       }
 
       return nullptr;
     }
 
-    llvm::Constant *getDefaultAssociatedConformanceAccessFunction(
+    void defineDefaultAssociatedConformanceAccessFunction(
                       AssociatedConformance requirement,
                       ProtocolConformanceRef conformance) {
       auto accessor =
@@ -839,7 +839,7 @@ namespace {
                                                     conformance.getConcrete());
         auto returnValue = conformanceI->getTable(IGF, &associatedTypeMetadata);
         IGF.Builder.CreateRet(returnValue);
-        return accessor;
+        return;
       }
 
       // For an abstract table, emit a reference to the witness table.
@@ -852,7 +852,7 @@ namespace {
             cast<ArchetypeType>(associatedTypeInContext),
             associatedProtocol);
       IGF.Builder.CreateRet(returnValue);
-      return accessor;
+      return;
     }
 
     void addAssociatedTypeNames() {