Completion fails if it's invoked on the last column of a line and the previous token is an identifier

The simplest repro is

```cpp
#define FOO 
FOO <invoke completion>
```

<img width="490" height="180" alt="Image" src="https://github.com/user-attachments/assets/3c294571-4026-4237-898d-9f1b15cd6325" />

This was discovered from a user report at https://github.com/microsoft/vscode-cpptools/issues/13634 .