[TCling] Do not add decls for control statements if already annotated

[devajithvs]

This Pull request:

Changes or fixes:

Checklist:

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

This PR fixes https://github.com/root-project/root/issues/8367

[github-actions[bot]]

Test Results

    11 files      11 suites   2d 6h 1m 33s :stopwatch:  2 631 tests  2 631 :white_check_mark: 0 :zzz: 0 :x: 27 316 runs  27 316 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 1b3bab61.

:recycle: This comment has been updated with latest results.

[devajithvs]

Not a proper solution.

What we ideally want to do is check if a VarDecl with the same identifier is already added for the current ControlScope.

[dpiparo]

I also think a test would be good. @vgvassilev in which subdirectory would you put the simple macro which was crashing in https://github.com/root-project/roottest/tree/master/cling ?

[vgvassilev]

Maybe we can do a death test in the test folder of core/metacling.