Make helper functions static

NFC.

llvm-svn: 231657
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index eeed7db..a421ee6 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -8187,8 +8187,8 @@
   return !isLowercase(str.front());
 }
 
-Optional<int> GetNSMutableArrayArgumentIndex(Sema &S, ObjCMessageExpr *Message) {
-
+static Optional<int> GetNSMutableArrayArgumentIndex(Sema &S,
+                                                    ObjCMessageExpr *Message) {
   if (S.NSMutableArrayPointer.isNull()) {
     IdentifierInfo *NSMutableArrayId =
       S.NSAPIObj->getNSClassId(NSAPI::ClassId_NSMutableArray);