SIL: Use only interface types in the verifier.

Treat the interface types of SILFunctionTypes as the canonical representation in the verifier. Do a bunch of supporting and annoyingly irreducible work to enable this:

- Stop trying to uncurry generic parameter lists during type lowering and preserve the structure of AST GenericParamLists. This makes mapping dependent types into contexts easier.
- Properly walk generic parameter lists at all depths when grooming substitution vectors for use with substGenericArgs interfaces.
- Reseat the generic parameter lists created for protocol_method results so that we don't expect the outer Self archetype to be unbound; it's provided by the extra data of the result.
- Hack SILFunctionType serialization never to use a decl reference when serializing its generic param list. When this happens, we get incorrect archetypes. This is a gross hack, but when we're able to jump all the way to interface types, it can go away.

Putting these ducks in a row nicely un-XFAILs TextFormatting.swift.

Swift SVN r11989

lib/SILGen/SILGenDecl.cpp

@@ -118,8 +118,7 @@ namespace {
 ArrayRef<Substitution> SILGenFunction::getForwardingSubstitutions() {
   auto outerFTy = F.getLoweredFunctionType();
   if (outerFTy->isPolymorphic()) {
-    return buildForwardingSubstitutions(
-                            outerFTy->getGenericParams()->getAllArchetypes());
+    return buildForwardingSubstitutions(outerFTy->getGenericParams());
   }
   return {};
 }
@@ -2053,7 +2052,15 @@ SILGenModule::emitProtocolWitness(ProtocolConformance *conformance,
                                  SourceLoc(),
                                  reqtParams.getRequirements(),
                                  SourceLoc());
-    methodParams->setOuterParameters(conformanceParams);
+    
+    // Preserve the depth of generic arguments by adding an empty outer generic
+    // param list if the conformance is concrete.
+    GenericParamList *outerParams = conformanceParams;
+    if (!outerParams)
+      outerParams = GenericParamList::create(getASTContext(),
+                                             SourceLoc(), {}, SourceLoc());
+      
+    methodParams->setOuterParameters(outerParams);
     methodParams->setAllArchetypes(reqtParams.getAllArchetypes());
     methodTy = CanPolymorphicFunctionType::get(pft.getInput(), pft.getResult(),
                                                methodParams,