Revert "[libclang] Always Dup in createRef(StringRef) (#125020)"

This reverts commit e76739e.

Author: vitalybuka

clang/docs/ReleaseNotes.rst

@@ -257,9 +257,6 @@ clang-format
 libclang
 --------
 
-- Fixed a buffer overflow in ``CXString`` implementation. The fix may result in
-  increased memory allocation.
-
 Code Completion
 ---------------
 

clang/tools/libclang/CXString.cpp

@@ -87,7 +87,19 @@ CXString createRef(StringRef String) {
   if (String.empty())
     return createEmpty();
 
-  return createDup(String);
+  // If the string is not nul-terminated, we have to make a copy.
+
+  // FIXME: This is doing a one past end read, and should be removed! For memory
+  // we don't manage, the API string can become unterminated at any time outside
+  // our control.
+
+  if (String.data()[String.size()] != 0)
+    return createDup(String);
+
+  CXString Result;
+  Result.data = String.data();
+  Result.private_flags = (unsigned) CXS_Unmanaged;
+  return Result;
 }
 
 CXString createDup(StringRef String) {