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

[swote-git]

Background

This issue is filed based on a suggestion by @Endilll during a code review.

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:

#if 1, 2
#endif

Clang currently produces a diagnostic like this:

<stdin>:1:6: error: expected end of line in preprocessor expression
    1 | #if 1, 2
      |      ^
<stdin>: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.

[Endilll]

Another test case from the same PR highlights a similar diagnostics that should be improved:

// Test 7: Comma in #embed limit parameter (C++23+)
#if __cplusplus >= 202302L
// since-cxx23-error@+1 {{expected ')'}}
#embed "jk.txt" limit(1, 2)
#endif