[clang] [clang][test] Add test for incompatible cv-qualified reference types in conversion function template (PR #81950)

[Duo Wang via cfe-commits]

wdunicornpro wrote:

@zygoloid Sorry about the abrupt ping but I figured you might have the best knowledge about that part of code. Do you have any thoughts on this?

https://github.com/llvm/llvm-project/pull/81950