[llvm] Create a CharSetConverter class with both iconv and icu support (PR #74516)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

One minor comment.  I think I'm satisfied this is doing everything it's supposed to otherwise.  Probably someone else should check the CMake stuff, though; I'm not confident in my understanding of CMake syntax.

https://github.com/llvm/llvm-project/pull/74516