[clang] [clang-repl] fix segfault in CleanUpPTU() (PR #75629)

[Pavel Kalugin via cfe-commits]

p4vook wrote:

I converted the PR to draft, as this seems to be a part of a bigger problem with inserting statements into wrong nodes of the syntax tree (this seems to only show up when namespaces are present). I believe it shouldn't be fixed just by a simple check for `nullptr`.

https://github.com/llvm/llvm-project/pull/75629