[clang][Preprocessor] Improve diagnostic message for top-level comma in #if expressions

### Background

This issue is filed based on a suggestion by @Endilll during a code [review](https://github.com/llvm/llvm-project/pull/155570#discussion_r2368814335).

The current diagnostic for a top-level comma in a preprocessor expression is not very actionable, as it points to the end of the line instead of the problematic token.

---

### Current Behavior

Given the following code:
```c
#if 1, 2
#endif
```

Clang currently produces a diagnostic like this:
```c
:1:6: error: expected end of line in preprocessor expression
1 | #if 1, 2
| ^
:1:2: error: unterminated conditional directive
1 | #if 1, 2
| ^
```

A better diagnostic would be more directly that commas should not be included. Rather than just indicating EOL.