[libclang] Always Dup in createRef(StringRef) (#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
#2 in DumpCXCommentInternal llvm-project/clang/tools/c-index-test/c-index-test.c:674:7
#3 in DumpCXCommentInternal llvm-project/clang/tools/c-index-test/c-index-test.c:674:7
#4 in DumpCXComment llvm-project/clang/tools/c-index-test/c-index-test.c:685:3
#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
#2 in Allocate llvm-project/llvm/include/llvm/Support/Allocator.h:216:12
#3 in Allocate llvm-project/llvm/include/llvm/Support/AllocatorBase.h:53:43
#4 in Allocate<char> llvm-project/llvm/include/llvm/Support/AllocatorBase.h:76:29
#5 in convertCodePointToUTF8 llvm-project/clang/lib/AST/CommentLexer.cpp:42:30
#6 in clang::comments::Lexer::resolveHTMLDecimalCharacterReference(llvm::StringRef) const llvm-project/clang/lib/AST/CommentLexer.cpp:76:10
#7 in clang::comments::Lexer::lexHTMLCharacterReference(clang::comments::Token&) llvm-project/clang/lib/AST/CommentLexer.cpp:615:16
#8 in consumeToken llvm-project/clang/include/clang/AST/CommentParser.h:62:9
#9 in clang::comments::Parser::parseParagraphOrBlockCommand() llvm-project/clang/lib/AST/CommentParser.cpp
#10 in clang::comments::Parser::parseFullComment() llvm-project/clang/lib/AST/CommentParser.cpp:925:22
#11 in clang::RawComment::parse(clang::ASTContext const&, clang::Preprocessor const*, clang::Decl const*) const llvm-project/clang/lib/AST/RawCommentList.cpp:221:12
#12 in clang::ASTContext::getCommentForDecl(clang::Decl const*, clang::Preprocessor const*) const llvm-project/clang/lib/AST/ASTContext.cpp:714:35
#13 in clang_Cursor_getParsedComment llvm-project/clang/tools/libclang/CXComment.cpp:36:35
#14 in PrintCursorComments llvm-project/clang/tools/c-index-test/c-index-test.c:756:25
```
