I'm mostly developing in Linux, so I'm using Vim as main editor. That (and Emacs) in the only editor I know that really handles big codebases well.
I'm sure many Clang/LLVM developers are using Xcode on a Mac, so that should be fine as well.
For C2 development, you don't really need to go into LLVM/Clang code that well. Just following the INSTALL.md in the project root does the trick.
Only when changing the Tokenizer or adding/changing diagnostic messages are changes to Clang needed. No changes were needed in LLVM at all.
I've discussed merging the C2 changes upstream, some Clang developers were (quite surprisingly) open to it. Clang and LLVM have a great
development ecosystem with a good vibe.