[clang] [clang-repl] Fix assertion failure in CleanUpPTU() (PR #85378)

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

I can reproduce it downstream in the implicit Undo after including a header that causes a lot of parse errors in nested headers. I am afraid it's unpractical to emulate in the test suite upstream.

The code is exercised in all Undo tests. They don't reproduce the issue right now, but it's just a matter of likeliness. I think this fix is fine without an additional test.

https://github.com/llvm/llvm-project/pull/85378