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

[via cfe-commits]

isuckatcs wrote:

> may be instantiated even if unused

Hmm, this is a bit vague I think. The user might ask when that instantiation happens, or if they have a control over it. We should probably mention that it is decided by the compiler implementation.

What about the following version?

_When a class template is instantiated, the instantiation of its unused virtual member functions can vary by compiler implementations._

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