AST: Remove TypeDecl::getDeclaredType()

A pointless use of polymorphism -- the result values are not
interchangeable in any practical sense:

- For GenericTypeParamDecls, this returned getDeclaredInterfaceType(),
  which is an interface type.

- For AssociatedTypeDecls, this returned the sugared AssociatedTypeType,
  which desugars to an archetype.

- For TypeAliasDecls, this returned TypeAliasDecl::getAliasType(),
  which desugars to a type containing archetypes.

- For NominalTypeDecls, this returned NominalTypeDecl::getDeclaredType(),
  which is the unbound generic type, a special case used for inferring
  generic arguments when they're not written in source.

lib/SILGen/SILGenFunction.cpp

@@ -518,9 +518,9 @@ void SILGenFunction::emitArtificialTopLevel(ClassDecl *mainClass) {
     // we're getting away with it because the types are guaranteed to already
     // be imported.
     ASTContext &ctx = getASTContext();
-    Module *UIKit = ctx.getLoadedModule(ctx.getIdentifier("UIKit"));
+    DeclContext *UIKit = ctx.getLoadedModule(ctx.getIdentifier("UIKit"));
     SmallVector<ValueDecl *, 1> results;
-    UIKit->lookupQualified(UIKit->getDeclaredType(),
+    UIKit->lookupQualified(UIKit->getDeclaredInterfaceType(),
                            ctx.getIdentifier("UIApplicationMain"),
                            NL_QualifiedDefault,
                            /*resolver*/nullptr,