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/AST/ASTContext.cpp

@@ -1487,6 +1487,10 @@ SILFunctionType::SILFunctionType(GenericParamList *genericParams, ExtInfo ext,
     auto getArchetypesAsDependentTypes = [&](Type t) -> Type {
       if (!t) return t;
       if (auto arch = t->getAs<ArchetypeType>()) {
+        // As a kludge, we allow Self archetypes of protocol_methods to be
+        // unapplied.
+        if (arch->getSelfProtocol() && !archetypeMap.count(arch))
+          return arch;
         return arch->getAsDependentType(archetypeMap);
       }
       return t;