[SYCL] Keep platform_impl's device_impls alive until shutdown

[aelovikov-intel]

After that devices are never destroyed until the SYCL RT library shutdown. In practice, that means that before the change a simple

int main() { sycl::device d; }

went into platform ctor, then queried all the platform's devices to check that it has some, returned from ctor and those sycl::devices created on stack were already destroyed. After that, when creating user's sycl::device d we were re-creating device hierarchy for the platform at SYCL level again (including some calls to urDeviceGetInfo during device_impl creation).

After the changes, devices created when veryfing that platform isn't empty are preserved inside the platform_impl object and this existing SYCL devices hierarchy is used when creating user's device object.

A note on the implementation: device_impl has an std::shared_ptr<platform_impl> inside so we can't rely on automatic resource management just by the nature of std::shared_ptr everywhere (and we haven't changed this aspect in #18143). As such, we have to perform some explicit resource release during shutdown procedure (or in ~UrMock() for unittests).