My recent refactoring of default arguments for the memberwise initializer
accidentally dropped support for getting a default argument when the
attached property wrapper has an init(). Reinstate that support,
fixing rdar://problem/52116923.
When the backing storage of a wrapped property is default-initialized via the
property wrapper type's init(), don't count that as a direct initialization
of the backing storage for the purposes of constructing the memberwise
initializer. Instead, treat this case the same as if there were no initializer,
keying the form of the memberwise initializer off the presence of
init(initialValue:).
The determination of which type to use within the implicit memberwise
initializer for a wrapper property (between the original property type
and the wrapper type) could be affected by the synthesis of the call
to the default initializer. Rewrite the logic here to depend on
aspects of the AST that don't change with further type checking.
