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

We can't guaranty that underlying string is
0-terminated and [String.size()] is even in the
same allocation.


https://lab.llvm.org/buildbot/#/builders/94/builds/4152/steps/17/logs/stdio
```
==c-index-test==1846256==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0  in clang::cxstring::createRef(llvm::StringRef) llvm-project/clang/tools/libclang/CXString.cpp:96:36
    #1  in DumpCXCommentInternal llvm-project/clang/tools/c-index-test/c-index-test.c:521:39
    llvm#2  in DumpCXCommentInternal llvm-project/clang/tools/c-index-test/c-index-test.c:674:7
    llvm#3  in DumpCXCommentInternal llvm-project/clang/tools/c-index-test/c-index-test.c:674:7
    llvm#4  in DumpCXComment llvm-project/clang/tools/c-index-test/c-index-test.c:685:3
    llvm#5  in PrintCursorComments llvm-project/clang/tools/c-index-test/c-index-test.c:768:7

  Memory was marked as uninitialized
    #0  in __msan_allocated_memory llvm-project/compiler-rt/lib/msan/msan_interceptors.cpp:1023:5
    #1  in Allocate llvm-project/llvm/include/llvm/Support/Allocator.h:172:7
    llvm#2  in Allocate llvm-project/llvm/include/llvm/Support/Allocator.h:216:12
    llvm#3  in Allocate llvm-project/llvm/include/llvm/Support/AllocatorBase.h:53:43
    llvm#4  in Allocate<char> llvm-project/llvm/include/llvm/Support/AllocatorBase.h:76:29
    llvm#5  in convertCodePointToUTF8 llvm-project/clang/lib/AST/CommentLexer.cpp:42:30
    llvm#6  in clang::comments::Lexer::resolveHTMLDecimalCharacterReference(llvm::StringRef) const llvm-project/clang/lib/AST/CommentLexer.cpp:76:10
    llvm#7  in clang::comments::Lexer::lexHTMLCharacterReference(clang::comments::Token&) llvm-project/clang/lib/AST/CommentLexer.cpp:615:16
    llvm#8  in consumeToken llvm-project/clang/include/clang/AST/CommentParser.h:62:9
    llvm#9  in clang::comments::Parser::parseParagraphOrBlockCommand() llvm-project/clang/lib/AST/CommentParser.cpp
    llvm#10 in clang::comments::Parser::parseFullComment() llvm-project/clang/lib/AST/CommentParser.cpp:925:22
    llvm#11 in clang::RawComment::parse(clang::ASTContext const&, clang::Preprocessor const*, clang::Decl const*) const llvm-project/clang/lib/AST/RawCommentList.cpp:221:12
    llvm#12 in clang::ASTContext::getCommentForDecl(clang::Decl const*, clang::Preprocessor const*) const llvm-project/clang/lib/AST/ASTContext.cpp:714:35
    llvm#13 in clang_Cursor_getParsedComment llvm-project/clang/tools/libclang/CXComment.cpp:36:35
    llvm#14 in PrintCursorComments llvm-project/clang/tools/c-index-test/c-index-test.c:756:25
 ```

Author: vitalybuka

clang/docs/ReleaseNotes.rst

@@ -252,6 +252,9 @@ 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

@@ -87,19 +87,7 @@ CXString createRef(StringRef String) {
   if (String.empty())
     return createEmpty();
 
-  // 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;
+  return createDup(String);
 }
 
 CXString createDup(StringRef String) {