Remove uses of llvm::empty

It's deprecated in rebranch

Author: eeckstein

include/swift/SILOptimizer/Analysis/ClosureScope.h

@@ -173,7 +173,7 @@ class ClosureFunctionOrder {
 
   ArrayRef<SILFunction *> getTopDownFunctions() const {
     assert(!topDownFunctions.empty()
-           || llvm::empty(csa->getModule()->getFunctions()));
+           || csa->getModule()->getFunctions().empty());
     return topDownFunctions;
   }
 

lib/AST/Attr.cpp

@@ -689,7 +689,7 @@ static void printDifferentiableAttrArguments(
             return false;
         return true;
       });
-  if (!llvm::empty(requirementsToPrint)) {
+  if (!requirementsToPrint.empty()) {
     if (!isLeadingClause)
       stream << ' ';
     stream << "where ";