[clang] [clang][Diagnostic] Clarify error message for auto (PR #149781)

[Baranov Victor via cfe-commits]

vbvictor wrote:

Hi, thank you for your contribution!
Please add comprehensive test-cases for your change to make sure everything work as expected and continue to do so in the future.
Please add an entry in `clang/docs/ReleaseNotes.rst` to let everyone know about your fix.
Please add "Fixes https://github.com/llvm/llvm-project/issues/147324" into PR description to automatically link issue to this PR.

https://github.com/llvm/llvm-project/pull/149781