Revert "[libclang] Always Dup in createRef(StringRef)" (#127076)

Reverts llvm/llvm-project#125020


https://lab.llvm.org/buildbot/#/builders/24/builds/5252/steps/12/logs/stdio

```
==c-index-test==2512295==ERROR: AddressSanitizer: heap-use-after-free on address 0xe19338c27992 at pc 0xc66be4784830 bp 0xe0e33660df00 sp 0xe0e33660d6e8
READ of size 23 at 0xe19338c27992 thread T1
    #0 0xc66be478482c in printf_common(void*, char const*, std::__va_list) /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors_format.inc:563:9
    #1 0xc66be478643c in vprintf /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:1699:1
    #2 0xc66be478643c in printf /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:1757:1
    #3 0xc66be4839384 in FilteredPrintingVisitor /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/clang/tools/c-index-test/c-index-test.c:1359:5
    #4 0xe4e3454f12e8 in clang::cxcursor::CursorVisitor::Visit(CXCursor, bool) /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/clang/tools/libclang/CIndex.cpp:227:11
    #5 0xe4e3454f48a8 in bool clang::cxcursor::CursorVisitor::visitPreprocessedEntities<clang::PreprocessingRecord::iterator>(clang::PreprocessingRecord::iterator, clang::PreprocessingRecord::iterator, clang::PreprocessingRecord&, clang::FileID) CIndex.cpp
    
0xe19338c27992 is located 82 bytes inside of 105-byte region [0xe19338c27940,0xe19338c279a9)
freed by thread T1 here:
    #0 0xc66be480040c in free /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:51:3
    #1 0xc66be4839728 in GetCursorSource c-index-test.c
    #2 0xc66be4839368 in FilteredPrintingVisitor /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/clang/tools/c-index-test/c-index-test.c:1360:12
    #3 0xe4e3454f12e8 in clang::cxcursor::CursorVisitor::Visit(CXCursor, bool) /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/clang/tools/libclang/CIndex.cpp:227:11
    #4 0xe4e3454f48a8 in bool clang::cxcursor::CursorVisitor::visitPreprocessedEntities<clang::PreprocessingRecord::iterator>(clang::PreprocessingRecord::iterator, clang::PreprocessingRecord::iterator, clang::PreprocessingRecord&, clang::FileID) CIndex.cpp


previously allocated by thread T1 here:
    #0 0xc66be4800680 in malloc /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:67:3
    #1 0xe4e3456379b0 in safe_malloc /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/llvm/include/llvm/Support/MemAlloc.h:26:18
    #2 0xe4e3456379b0 in createDup /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/clang/tools/libclang/CXString.cpp:95:40
    #3 0xe4e3456379b0 in clang::cxstring::createRef(llvm::StringRef) /home/b/sanitizer-aarch64-linux-bootstrap-asan/build/llvm-project/clang/tools/libclang/CXString.cpp:90:10
```

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) {