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.
Currently, tests are scattered throughout multiple directories, making
it difficult to determine what is tested and what is not tested.
With upcoming work on interoperability between Swift and C++
(https://forums.swift.org/t/manifesto-interoperability-between-swift-and-c/33874),
we'd like to rearrange the tests and bring tests covering a single C/C++
feature next to each other.
This PR also adds tests for enabling C++ interop where possible.
