The Swift 5 struct layout algorithm is to lay out the stored properties using naive, in-order C layout. Class layout is the same, except the first field is either the base class (recursively) or a fixed header (a pointer plus 64 unaligned bits). The only significant difference from C, besides the lack of bit-fields, is that we distinguish size vs. stride and so don't round a struct (or base class's) size up to the maximum alignment of any stored property. But you can also look at lib/IRGen/StructLayout.cpp in the source code.
The ABI rule for tuples always uses the Swift 5 layout algorithm, and that is a permanent guarantee. For structs, officially the layout algorithm is determined by an algorithm that's somehow parameterized by the struct and the build settings; however, right now that algorithm is always "use the Swift 5 layout algorithm".