[clang] [clang][NFC] Add tests for static_cast<T&&> of a bit-field lvalue. (PR #231092)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

It's a bit confusing to use the standard section label in a link while the link is actually to cppreference. We use the PR description as squashed commit message by default, and perhaps it's better to avoid the confusing link in the squashed commit.

Would you mind to use https://timsong-cpp.github.io/cppwp/n4950/expr.static.cast#3 instead?

https://github.com/llvm/llvm-project/pull/231092