[libc++] Improve diagnostic when violating `std::atomic` trivially copyable mandates (#131754)

When attempting to instantiate `std::atomic` with a non trivially
copyable type, one gets errors from instantiating internals before the
actual static assertion that check the template parameter type
requirements.

The `verify` test for it had a `// ADDITIONAL_COMPILE_FLAGS: -Xclang
-verify-ignore-unexpected=error` directive to work around this issue.
The changes I propose enable us to drop that directive.
As I understand it, the `verify` test was misplaced so I moved it to
`test/{std -> libcxx}/atomics`.

(I ran into this while working on #121414 in which we would add another
static assertion in `__check_atomic_mandates`)

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