[clang] [libcxx] [llvm] [Clang] Add warnings when mixing different charN_t types (PR #138708)

[Qinkun Bao via cfe-commits]

qinkunbao wrote:

Hi, this PR broke a few buildbots. Could you take a look?

https://lab.llvm.org/buildbot/#/builders/66/builds/13997
https://lab.llvm.org/buildbot/#/builders/51/builds/16212


https://github.com/llvm/llvm-project/pull/138708