std::function -> llvm::function_ref. NFC.

llvm-svn: 312336
diff --git a/clang/lib/Analysis/CloneDetection.cpp b/clang/lib/Analysis/CloneDetection.cpp
index 637e042..8c833f4 100644
--- a/clang/lib/Analysis/CloneDetection.cpp
+++ b/clang/lib/Analysis/CloneDetection.cpp
@@ -477,7 +477,8 @@
 
 void CloneConstraint::splitCloneGroups(
     std::vector<CloneDetector::CloneGroup> &CloneGroups,
-    std::function<bool(const StmtSequence &, const StmtSequence &)> Compare) {
+    llvm::function_ref<bool(const StmtSequence &, const StmtSequence &)>
+        Compare) {
   std::vector<CloneDetector::CloneGroup> Result;
   for (auto &HashGroup : CloneGroups) {
     // Contains all indexes in HashGroup that were already added to a