[clang] Migrate DR tests to `static_assert` (#88611)

This patch touches a number of tests that run in C++98 mode that have
been using array size as a context that requires a constant expression,
replacing it with a `static_assert` backported via a macro. This reduces
noise in expected directives that comes from diagnostics around VLAs.

This patch also showcases that DR tests would benefit from folding in
constant expressions in C++98 mode, but I'm not sure it's even on the
table. If it is, I'd be happy to prepare a PR for that, and rebase this
PR on top of it.

CC @AaronBallman