This flag was added back in 2020, but it didn't function properly, since a lot of other code in the compiler assumed the platform-default C++ stdlib until recently (https://github.com/swiftlang/swift/pull/75589).
The recommended way to use a non-default C++ stdlib in Swift now is to pass `-Xcc -stdlib=xyz` argument to the compiler.
This change removes the `-experimental-cxx-stdlib` flag.
This doesn't yet allow including C++ headers on platforms where libc++
isn't the default; see comments in UnixToolChains.cpp for details.
However, it does, for example, allow throwing and catching exceptions in C++
code used through interop, unblocking
https://github.com/apple/swift/pull/30674/files.
The flags (-enable-experimental-cxx-interop and -experimental-cxx-stdlib) carry
"experimental" in the name to emphasize that C++ interop is still an
experimental feature.
Co-authored-by: Michael Forster <forster@google.com>
