CodeGen: mark ObjC cstring literals as unnamed_addr

These are all emitted into a section with a cstring_literal attribute.  The
attribute permits the linker to coalesce the string contents.  The address of
the strings are not important.

llvm-svn: 281855
diff --git a/clang/test/CodeGenObjCXX/encode.mm b/clang/test/CodeGenObjCXX/encode.mm
index 26d682d..5bb4a1e 100644
--- a/clang/test/CodeGenObjCXX/encode.mm
+++ b/clang/test/CodeGenObjCXX/encode.mm
@@ -213,7 +213,7 @@
   dynamic_class dynamic_class_ivar;
 }
 @end
-// CHECK: private constant [41 x i8] c"{dynamic_class=\22_vptr$dynamic_class\22^^?}\00"
+// CHECK: private unnamed_addr constant [41 x i8] c"{dynamic_class=\22_vptr$dynamic_class\22^^?}\00"
 
 namespace PR17142 {
   struct A { virtual ~A(); };