We synthesize a Swift function that only calls a C++ funtion that
produces the default argument. We produce this function for each module
that imports the C++ function with the default argument. This symbol
needs to be public as it is created in the context of the Clang module
and used from the Swift module. To avoid the linker errors, we always
emit this function into the client which is also the right thing to do
as the whole body is a single function call.
rdar://138513915
