[clang] [clang][CodeGen] Identify virtual methods by their override chain root in the call graph section (PR #230732)

[Prabhu Rajasekaran via cfe-commits]

Prabhuk wrote:

Superseded by #230831 (same change, same commit), recreated from the in-repo branch `users/Prabhuk/callgraph/1-virtual-methods` so that #230832 can be stacked on it natively (GitHub stack #230833).

https://github.com/llvm/llvm-project/pull/230732