[clang][Tooling] Fix potential UB in ExpandResponseFilesCompilationDatabase

[lh123]

Summary

vector::assign will cause UB at here.

fixes: https://github.com/clangd/clangd/issues/223

[merge_guards_bot]

Build result: pass - 60602 tests passed, 0 failed and 726 were skipped.

Log files: console-log.txt, CMakeCache.txt

[sammccall]

Thanks!
Might be worth a comment like "don't assign directly, Argv aliases CommandLine" or so

[lh123]

Address comments.

[lh123]

@sammccall Could you commit this, I don't have commit access.

[merge_guards_bot]

Build result: pass - 60606 tests passed, 0 failed and 726 were skipped.

Log files: console-log.txt, CMakeCache.txt

[merge_guards_bot]

Build result: pass - 60606 tests passed, 0 failed and 726 were skipped.

Log files: console-log.txt, CMakeCache.txt

clang/lib/Tooling/ExpandResponseFilesCompilationDatabase.cpp

for (auto &Cmd : Cmds) {
         SeenRSPFile |= Arg.front() == '@';
       }
       if (!SeenRSPFile)
         continue;
       llvm::BumpPtrAllocator Alloc;
       llvm::StringSaver Saver(Alloc);
       llvm::cl::ExpandResponseFiles(Saver, Tokenizer, Argv, false, false, *FS,
                                     llvm::StringRef(Cmd.Directory));
-      Cmd.CommandLine.assign(Argv.begin(), Argv.end());
+      // Don't assign directly, Argv aliases CommandLine.
+      std::vector<std::string> ExpandedArgv(Argv.begin(), Argv.end());
+      Cmd.CommandLine = std::move(ExpandedArgv);
     }
     return Cmds;
   }
 
 private:
   std::unique_ptr<CompilationDatabase> Base;
   llvm::cl::TokenizerCallback Tokenizer;
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS;