Add apple-clang 15

[AbrilRBS]

Changelog: Feature: Add apple-clang 15.
Docs: Omit

I recently downloaded XCode 15 (Beta 3) to try some things, found that after a conan profile detect I had 15 which was not supported by default.

[memsharded]

Sounds good.
Did you check that apple-clang 15 doesn't change anything in term of default cppstd, like any change necessary in _apple_clang_supported_cppstd or similar?

[AbrilRBS]

Yes! The team suggested that I should look into it and that C++ versions were usually the reason that updates were delayed, so I made sure that: The default C++ version hasn't changed, it's still 98 if not specified, and no new c++ flag is available (c++2c might be, but we don't add those so didn't even check)