Textual interfaces for 'Darwin' built with recent compilers specify that it is built witout C++ interop enabled. However, to ensure compatibility with versions of the 'Darwin' module built with older compilers, we hard-code this fact. This is required to break the module cycle that occurs when building the 'Darwin' module with C++ interop enabled, where the underlying 'Darwin' clang module depends on C++ standard library for which the compiler brings in the 'CxxStdlib' Swift overlay, which depends on 'Darwin'.
When we discover a textual module dependency which is a module which was not originally built from source using C++ interop (specifying '-formal-cxx-interoperability-mode=off'), avoid looking up the C++ standard library Swift overlay for it. This is required for the case of the 'Darwin' module, for example, which includes headers which map to C++ stdlib headers when the compiler is operating in C++ interop mode, but the C++ standard library Swift overlay module itself depends on 'Darwin', which results in a cycle. To resolve such situations, we can rely on the fact that Swift textual interfaces of modules which were not built with C++ interop must be able to build without importing the C++ standard library Swift overlay, so we avoid specifying it as a dependency for such modules.
The primary source module, as well as Swift textual module dependencies which *were* built with C++ interop will continue getting a direct depedency of the 'CxxStdlib' Swift module.
Resolves rdar://150222155
