Requestify getInterfaceType() in Name Only

Make getInterfaceType() call validateDecl on behalf of its clients.  In effect, this makes the interface type behave like a request.  It also means that its clients no longer need to perform a number of undesirable anti-patterns in order to sidestep the bizarre API contract validateDecl has at the moment

In particular, the following things are no longer necessary:

- Checking for a missing interface type then validating
- Validating the interface type then retrieving it
- Validating the interface type then retrieving a derived value

These anti-patterns will be removed in follow-up commits

Author: CodaFi

lib/AST/Decl.cpp

@@ -2697,7 +2697,13 @@ bool ValueDecl::hasInterfaceType() const {
 }
 
 Type ValueDecl::getInterfaceType() const {
-  assert(hasInterfaceType() && "No interface type was set");
+  if (!hasInterfaceType()) {
+    // Our clients that don't register the lazy resolver are relying on the
+    // fact that they can't pull an interface type out to avoid doing work.
+    // This is a necessary evil until we can wean them off.
+    if (auto resolver = getASTContext().getLazyResolver())
+      resolver->resolveDeclSignature(const_cast<ValueDecl *>(this));
+  }
   return TypeAndAccess.getPointer();
 }
 
@@ -3241,7 +3247,7 @@ Type TypeDecl::getDeclaredInterfaceType() const {
         selfTy, const_cast<AssociatedTypeDecl *>(ATD));
   }
 
-  Type interfaceType = hasInterfaceType() ? getInterfaceType() : nullptr;
+  Type interfaceType = getInterfaceType();
   if (interfaceType.isNull() || interfaceType->is<ErrorType>())
     return interfaceType;