[Type checker] Validate not-yet-typed declarations we find via unqualified name lookup.

When using the ASTScope-based unqualified name lookup, we might find
things that are in scope but haven't been fully type-checked yet;
allow us to perform type checking.

Author: DougGregor

lib/Sema/TypeCheckConstraints.cpp

@@ -472,6 +472,8 @@ resolveDeclRefExpr(UnresolvedDeclRefExpr *UDRE, DeclContext *DC) {
     }
 
     ValueDecl *D = Result.Decl;
+    if (!D->hasType()) validateDecl(D);
+
     if (!D->hasType()) {
       assert(D->getDeclContext()->isLocalContext());
       if (!D->isInvalid()) {

test/NameBinding/scope_map_lookup.swift

@@ -104,3 +104,7 @@ protocol Fooable {
 
   var foo: Foo { get }
 }
+
+let a = b ; let b = a // expected-error{{could not infer type for 'a'}} 
+// expected-error@-1 {{'a' used within its own type}}
+// FIXME: That second error is bogus.