[Utils] Extract CollectDebugInfoForCloning from CloneFunctionInto (#114537)
Summary:
Consolidate the logic in a single function. We do an extra pass over
Instructions but this is necessary to untangle things and extract
metadata cloning in a future diff.
Test Plan:
```
$ ninja check-llvm-unit check-llvm
[211/213] Running the LLVM regression tests
Testing Time: 106.06s
Total Discovered Tests: 62601
Skipped : 17 (0.03%)
Unsupported : 2518 (4.02%)
Passed : 59911 (95.70%)
Expectedly Failed: 155 (0.25%)
[212/213] Running lit suite
Testing Time: 12.47s
Total Discovered Tests: 8474
Skipped: 17 (0.20%)
Passed : 8457 (99.80%)
```
Extracted from #109032 (commit 3) (there are more refactors and cleanups
in subsequent commits)