[cling] Reuse existing weak symbols, also from JIT:#12018

Axel-Naumann wants to merge 5 commits intoroot-project:masterroot-project/root:masterfrom

Axel-Naumann:cling-JIT-reuse-weak-JIT-symsAxel-Naumann/root:cling-JIT-reuse-weak-JIT-symsCopy head branch name to clipboard