[clang-repl] Fix remove invalidates iterators in CleanUpPTU() (#85378)

Using remove() on DeclContext::lookup_result list invalidates iterators.

This assertion failure was one (fortunate) symptom:
```
clang/include/clang/AST/DeclBase.h:1337: reference clang::DeclListNode::iterator::operator*() const: Assertion `Ptr && "dereferencing end() iterator"' failed.
```