This does not include subscript operators.
Before this is re-enabled operators need to be re-implemented. Right now they are the source of a lot of bugs. They cause frequent crashes and mis compiles. Also, templated operators insert a lot of names into global lookup which causes problems.
They also don't work on Windows.
Add an convert to the new `target-swiftxx-frontend` substitution in lit
to control the C++ interop enabling in Swift. This allows for a single
site which will enable control of both an overridden standard (for
testing multiple C++ standards) and simplify writing tests.
The C++ interop modules require C++ support. Explicitly require C++ as
a feature when building these modules. This has no impact on the
changes as all the tests enable C++ already.
