Make function static that didn't need linkage.

In r373247 I added a helper function, but neglected to make it static.

llvm-svn: 373268
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index 5f181fa..95b14e4 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -1399,7 +1399,7 @@
 }
 #endif
 
-NamedDecl* getLambdaCallOperatorHelper(const CXXRecordDecl &RD) {
+static NamedDecl* getLambdaCallOperatorHelper(const CXXRecordDecl &RD) {
   if (!RD.isLambda()) return nullptr;
   DeclarationName Name =
     RD.getASTContext().DeclarationNames.getCXXOperatorName(OO_Call);