[clang] [clang][Sema] Simplify err_init_conversion_failed diagnostic message for const variables (PR #82109)

[via cfe-commits]

halbi2 wrote:

Hello, I made patch #128614 that fixes a similar diagnostic with `-Wsign-compare`. But my patch is much simpler. I simply remove the cv qualifiers before printing. Why would such an approach not be appropriate in this case as well?

https://github.com/llvm/llvm-project/pull/82109