[clang] [NFC][analyzer] Replace `CheckerContext::generateSink` with `addSink` (PR #234351)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

> I wish it had a tldr at the top, but the title kinda told what to do alongside the diff itself.

Good point, I rewrote the PR description to put the actual changes at the beginning and then say "This is motivated by..." to justify it.

https://github.com/llvm/llvm-project/pull/234351