[clang-tools-extra] [clang-tidy] reword the diag messages of `TemplateVirtualMemberFunctionCheck` (PR #211122)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

Standardese is not user-friendly. I think the following is better:

When a class template is instantiated, its virtual member functions may be instantiated even if unused.

https://github.com/llvm/llvm-project/pull/211122