[clang-repl] Inconsitent flushing between out of process and in-process mode

I’m using an `x86_64-unknown-linux-gnu` machine
```
// out of process
anutosh491@vv-nuc:/build/anutosh491/llvm-project/build/bin$ ./clang-repl --oop-executor=$PWD/llvm-jitlink-executor
clang version 22.0.0git (https://github.com/anutosh491/llvm-project.git 81ad8fbc2bb09bae61ed59316468011e4a42cf47)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /build/anutosh491/llvm-project/build/bin
Build config: +unoptimized, +assertions
clang-repl> #include
clang-repl> std::string myname;
clang-repl> std::cin >> myname;
Anutosh
clang-repl> std::cout << "Your name is " << myname;
clang-repl>

// in process
anutosh491@vv-nuc:/build/anutosh491/llvm-project/build/bin$ ./clang-repl
clang-repl> #include
clang-repl> std::string myname;
clang-repl> std::cin >> myname;
Anutosh
clang-repl> clang-repl> std::cout << "Your name is " << myname << std::endl;
Your name is Anutosh
clang-repl>
```