Blind fix for AppleClang version and char8_t support check

[traceon]

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Changelog category:

• Non-significant (changelog entry is not required)

Detailed description:
Added checks for AppleClang versions to allow them if it is Xcode 10.2+. Main known issue was char8_t support, which was added in Clang 7 (enabled with -std=c++20 -fchar8_t) and is enabled by default starting Clang 8 if only -std=c++20 is specified, according to https://clang.llvm.org/cxx_status.html
This is blind fix, verified to work only on Xcode 11.4.1.

The correspondence between AppleClang and upstream Clang versions is guessed based on:

• https://en.wikipedia.org/wiki/Xcode#Xcode_7.0_-_11.x_(since_Free_On-Device_Development) (see LLVM version)
• https://github.com/apple/llvm-project/blob/5b5d130d46d8289292fee9a740edef22bf76e6d2/clang/CMakeLists.txt#L375-L387
• various comments on web suggesting this is right way of matching them