llcppsymg:refine class fetch & add test case

[luoliwoshang]

• Refactored the class name retrieval logic, now extracting it by accessing the cursor's semantic parent, and removed redundant logic
• supplement test case
• supplement clangutil's test case

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.57%. Comparing base (32f41a0) to head (6f55f2f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #788 +/- ## ======================================= Coverage 97.57% 97.57% ======================================= Files 20 20 Lines 5165 5165 ======================================= Hits 5040 5040 Misses 106 106 Partials 19 19 ```

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[xushiwei]

should resolve conflicts.

[luoliwoshang]

should resolve conflicts.

resolve conflicts , and need merge the https://github.com/goplus/llgo/pull/757 for llcppsigfetch & gogensig