[clang] [Clang] Do not warn on UTF-16 -> UTF-32 conversions. (PR #163927)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman nope, i want to backport it :)

Sure! Make sure the backport has the release note then. :-D

https://github.com/llvm/llvm-project/pull/163927