[clang] [clang-repl] Fix the process return code if diagnostics occurred. (PR #89879)

[Stefan Gränitz via cfe-commits]

https://github.com/weliveindetail approved this pull request.

I assume the return code is supposed to reflect only frontend errors? (See my note inline.) From reading the test it's not obvious what is interactive mode and what is non-interactive/batch mode. Might be worth a note. Otherwise, this LGTM.

https://github.com/llvm/llvm-project/pull/89879