This is an attribute that gets put on an import in library FooKit to
keep it from being a requirement to import FooKit. It's not checked at
all, meaning that in this form it is up to the author of FooKit to
make sure nothing in its API or ABI depends on the implementation-only
dependency. There's also no debugging support here (debugging FooKit
/should/ import the implementation-only dependency if it's present).
The goal is to get to a point where it /can/ be checked, i.e. FooKit
developers are prevented from writing code that would rely on FooKit's
implementation-only dependency being present when compiling clients of
FooKit. But right now it's not.
rdar://problem/48985979
This is unfortunate in that it makes the linker do extra work, but in
practice it probably doesn't matter much, and meanwhile it handles all
our problems with @inlinable.
Alternate solution to rdar://problem/39338239
This reverts commit ee6e190e09. It's not
sufficient to solve the problem, and the choices were to do something
more complicated, or just take a simple brute force approach. We're
going with the latter.
