Use function_ref to avoid allocation in std::function. NFC.

llvm-svn: 289433
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index fd0654b..6a9afc4 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -11816,7 +11816,8 @@
 
 void Sema::RefersToMemberWithReducedAlignment(
     Expr *E,
-    std::function<void(Expr *, RecordDecl *, FieldDecl *, CharUnits)> Action) {
+    llvm::function_ref<void(Expr *, RecordDecl *, FieldDecl *, CharUnits)>
+        Action) {
   const auto *ME = dyn_cast<MemberExpr>(E);
   if (!ME)
     return;