Bump macOS Xcode baseline to 11?

[Ilmari Lauhakangas]

Stephan Bergmann kirjoitti 17.2.2020 klo 12.59:
> The current Xcode version appears to be 11.3.1.  I have no idea whether 
> we would need/want to explicitly require Xcode 11.3 or be happy with 
> just Xcode 11, but if we bump anyway it would probably make sense to 
> bump to Xcode 11.3, just to be on the safe side.  (See the "whether 
> Xcode is new enough" check in configure.ac.)

For reference:
https://developer.apple.com/documentation/xcode_release_notes/xcode_11_release_notes
https://developer.apple.com/documentation/xcode_release_notes/xcode_11_3_1_release_notes

The cool thing is we would get support for std::any, ::optional and 
::variant (added in Xcode 10).

Regarding C++20 support, the support table here is incomplete, but does 
have some stuff added: https://en.cppreference.com/w/cpp/compiler_support
If someone feels like doing some C++ colouring book work in the Apple 
Clang column, go ahead :) or email me your evidence of supported things 
and I will do it.

Ilmari