[clang-tools-extra] [clang-tidy] Fix false-positives in misc-static-assert caused by non-constexpr variables (PR #77203)

[Justin Cady via cfe-commits]

https://github.com/justincady approved this pull request.

Context: I have limited experience writing AST matchers, but I saw [the request for reviewers](https://discourse.llvm.org/t/clang-tidy-18-reviewers-wanted/76108). I'm basing this review on that limited experience and the tests put in place with this diff.

On those grounds, these changes LGTM.

https://github.com/llvm/llvm-project/pull/77203