Check operator declaration binding in the first pass for FuncDecls.

This eliminates the pre-pass we were performing to bind the operator
decls, which was only a pre-pass because we used to bind unresolved
declaration references too early. In the process, fixed some bugs
(e.g., it wasn't checking methods at all) and improved the QoI with
Fix-Its and notes:

t.swift:2:6: error: prefix unary operator missing 'prefix' attribute
func ~~~(x : Float) {}
     ^
     [prefix] 
t.swift:1:17: note: prefix operator found here
operator prefix ~~~ {}
                ^



Swift SVN r7099

lib/AST/Module.cpp

@@ -238,12 +238,12 @@ Optional<OP_DECL *> lookupOperatorDeclForName(Module *M,
 
   auto *TU = dyn_cast<TranslationUnit>(M);
   if (!TU)
-    return nullptr;
+    return Nothing;
 
   // Look for an operator declaration in the current module.
   auto found = (TU->*OP_MAP).find(Name.get());
   if (found != (TU->*OP_MAP).end())
-    return found->getValue();
+    return found->getValue()? Optional<OP_DECL *>(found->getValue()) : Nothing;
   
   // Look for imported operator decls.
   
@@ -262,7 +262,7 @@ Optional<OP_DECL *> lookupOperatorDeclForName(Module *M,
   if (importedOperators.empty()) {
     // Cache the mapping so we don't need to troll imports next time.
     (TU->*OP_MAP)[Name.get()] = nullptr;
-    return nullptr;
+    return Nothing;
   }
   if (importedOperators.size() == 1) {
     // Cache the mapping so we don't need to troll imports next time.