Align platform::operator== and operator!= with SYCL 2020

[KornevNikita]

Describe the bug

Current impl:

llvm/sycl/include/sycl/platform.hpp

Lines 117 to 119 in a38decb

	bool operator==(const platform &rhs) const { return impl == rhs.impl; }
	bool operator!=(const platform &rhs) const { return !(*this == rhs); }

According to SYCL 2020 should be:

friend bool operator==(const T& lhs, const T& rhs) { /* ... */ }
friend bool operator!=(const T& lhs, const T& rhs) { /* ... */ }

Need to replace old operators: guard old by "#ifndef __INTEL_PREVIEW_BREAKING_CHANGES" and add new under "#else". Thus we'll switch to new API with the next major release.

To reproduce

1. Include a code snippet that is as short as possible
2. Specify the command which should be used to compile the program
3. Specify the command which should be used to launch the program
4. Indicate what is wrong and what was expected

Environment

• OS: [e.g Windows/Linux]
• Target device and vendor: [e.g. Intel GPU]
• DPC++ version: [e.g. commit hash or output of clang++ --version]
• Dependencies version: [e.g. the output of sycl-ls --verbose]

Additional context

No response

[KornevNikita]

@Robertkq FYI

[Robertkq]

Awesome, I'll finish my current 2 issues that are assigned to me and will start working on this one if not taken, no need to be held for me in case anyone else wants to contribute :)