[SYCL] Less shared_ptr for platform_impl

[aelovikov-intel]

GlobalHandler::MPlatformCache keeps (shared) ownership of platform_impl objects, so none of them could be destroyed until SYCL RT library shutdown/unload process. As such, using raw pointers/reference to platform_impl throughout the SYCL RT is totally fine and would avoid extra costs of std::shared_ptr. However, our unittests don't work the same way and lifetimes are actually managed by all the data member std::shared_ptrs (e.g., see CurrentDevice.cpp as the most obvious case), so simply switching all PlatformImplPtr to raw pointer/reference doesn't work right now. I think it will be possible with an ABI break if we'd remove all the shared pointers but the one in the GlobalHandler.

What I'm doing here instead is the following:

• Try to keep lifetime management the same, i.e., all data member pointers are still "smart"
• Inherit from std::enable_shared_from_this() so that we could pass raw references around and only actually create smart pointers when absolutely necessary.
• Change internal APIs to operate more using plain references

This should make it much harder to write something like this:

  // BAD: Should have been `auto &`, extra atomic counter
  // increment/decrement without it.
  auto PlatformImpl = d.getPlatformImplPtr();

[Copilot]

Pull Request Overview

This PR refactors the SYCL runtime to reduce unnecessary use of std::shared_ptr for platform_impl objects by replacing them with raw references or pointers in critical parts of the code, thereby reducing ownership overhead. The changes update several interfaces and call sites throughout the codebase.

• Changed the platform interface in tests and source files (e.g. printers.cpp, platform.cpp) to use platform_impl references.
• Updated device and program manager implementations to adjust method calls and data access to the new raw reference API.
• Refactored platform_impl.hpp/.cpp and related files to expose getSharedPtrToSelf() and return references instead of smart pointers.

Reviewed Changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sycl/test/gdb/printers.cpp	Updated comments to reflect the change from shared_ptr to references for platform_impl.
sycl/source/platform.cpp	Modified platform_impl acquisition to return a shared pointer via getSharedPtrToSelf() and updated related call sites.
sycl/source/device.cpp	Adjusted device_impl initialization to use the new platform_impl API with references.
sycl/source/detail/usm/usm_impl.cpp	Replaced arrow operator calls with dot operator calls on platform_impl references.
sycl/source/detail/program_manager/program_manager.cpp	Refactored access to platform_impl from pointer to reference in option handling and backend option retrieval.
sycl/source/detail/platform_impl.hpp/.cpp	Changed several methods in platform_impl to return references instead of shared_ptr, including updates to helper methods and caching.
Other Files (global_handler.hpp/.cpp, device_impl.hpp/.cpp, context_impl.hpp/.cpp, backend files, etc.)	Updated all affected call sites and interface definitions to conform with the new ownership model.