macports-libcxx: sync with the latest compatible version of LLVM

[aeiouaeiouaeiouaeiouaeiouaeiou]

Bitcoin 27.0 now uses the std::bit_width function, which was only properly ​implemented in LLVM 12.

​https://build.macports.org/builders/ports-10.13_x86_64-builder/builds/228575/steps/install-port/logs/stdio

In file included from addrman.cpp:10:
In file included from ./addrman.h:9:
In file included from ./netaddress.h:11:
./random.h:207:25: error: no member named 'bit_width' in namespace 'std'
        int bits = std::bit_width(range);
                   ~~~~~^

Also more:

In file included from addrdb.cpp:10:
In file included from ./addrdb.h:10:
./util/fs.h:65:30: error: no matching constructor for initialization of 'const std::u8string &' (aka 'const basic_string<char8_t> &')
        const std::u8string& utf8_str{std::filesystem::path::u8string()};
                             ^       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/local/include/libcxx/v1/string:791:40: note: candidate constructor not viable: no known conversion from 'std::string' (aka 'basic_string<char, char_traits<char>, allocator<char>>') to 'const allocator_type' (aka 'const std::allocator<char8_t>') for 1st argument
    _LIBCPP_INLINE_VISIBILITY explicit basic_string(const allocator_type& __a)
                                       ^