Apple clang is currently version 14, but Apple's libc++ is version 13,
and so lacks even more std::ranges support than previously reported.
Add more stub backports to work around this.
OS X still uses clang-14, which lacks sufficient std::ranges support for
recent Rebirth changes.
- Rewrite uses of std::ranges::SYMBOL to ranges::SYMBOL
- Add a stub header that, on gcc, provides for each SYMBOL a statement
`using std::ranges::SYMBOL;`, to delegate back to the standard library
implementation.
- On clang, define a minimal implementation of the required symbols,
without constraint enforcement. Compile-testing with gcc will catch
constraint violations.
Once OS X clang ships a standard library with the required features,
this stub header will be removed and the uses changed back to their full
names.
