[cling] Adapt and move closer to clang-repl autocomplete machinery

[devajithvs]

This Pull request:

This is a draft PR:

Autocomplete in cling was broken after 9cdffe0 since cling does autocomplete a bit differently from clang-repl

This will also help us get rid of an LLVM patch that was introduced in the update to LLVM20.

Changes or fixes:

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #

[github-actions]

Test Results

     3 files  +     3       3 suites  +3   13h 3m 14s ⏱️ + 13h 3m 14s
 3 745 tests + 3 745   3 742 ✅ + 3 742  0 💤 ±0  3 ❌ +3 
10 622 runs  +10 622  10 615 ✅ +10 615  0 💤 ±0  7 ❌ +7 

For more details on these failures, see this check.

Results for commit 0465e0c. ± Comparison against base commit 7e8e273.