Follow up to https://github.com/llvm/llvm-project/pull/111902.
Makes sure all the `no_unique_address` tests are in the same place and
we don't rely on the host target triple (which means we don't need to
account for `[[msvc::no_unique_address]]` on Windows).
Now that we don't compile with the host compiler, this patch also adds
`-c` to the compilation command since we don't actually need the linked
binary in the test anyway (and on Darwin linking through Clang requires
the `xcrun` prefix to set up the SDK paths, etc.). We already do this in
`no_unique_address-with-bitfields.cpp` anyway.
