Add @_implementationOnly

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

lib/Serialization/SerializedModuleLoader.cpp

@@ -487,8 +487,10 @@ void swift::serialization::diagnoseSerializedASTLoadFailure(
         loadedModuleFile->getDependencies().begin(),
         loadedModuleFile->getDependencies().end(), std::back_inserter(missing),
         [&duplicates](const ModuleFile::Dependency &dependency) -> bool {
-          if (dependency.isLoaded() || dependency.isHeader())
+          if (dependency.isLoaded() || dependency.isHeader() ||
+              dependency.isImplementationOnly()) {
             return false;
+          }
           return duplicates.insert(dependency.RawPath).second;
         });