[clang] [clang-repl] Fix inconsistent flushing between in-process and out-of-process (PR #166368)

[Anutosh Bhat via cfe-commits]

anutosh491 wrote:

Approach 2: As what the PR has, configures the executor’s output streams to be unbuffered once at startup, ensuring immediate flushing for all subsequent inputs.
This is cleaner and faster than approach 1, which redundantly parsed and executed a flush command after every input line followed by an undo.

https://github.com/llvm/llvm-project/pull/166368