[clang-tools-extra] [clang-tidy] Improve readability-enum-initial-value diagnostic message (PR #176485)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

It would be better to emit diagnostics with `DiagnosticIDs::Note` to tell which and where enumerators are not initialized. Concatenating makes the message too long.

An optimization: For contiguous uninitialized enumerators, emit diagnostic messages for only the first and the last enumerator.

https://github.com/llvm/llvm-project/pull/176485