SourceKit: do not print implementation-only imports

Implementation-only imports are unnecessary in generated module interfaces, since they aren't exported to the module's dependencies, and the module's public API cannot refer to symbols imported as implementation-only.

Author: egorzhdan

lib/IDE/ModuleInterfacePrinting.cpp

@@ -607,6 +607,9 @@ void swift::ide::printModuleInterface(
     }
 
     auto ShouldPrintImport = [&](ImportDecl *ImportD) -> bool {
+      if (ImportD->getAttrs().hasAttribute<ImplementationOnlyAttr>())
+        return false;
+
       if (!TargetClangMod)
         return true;
       if (ImportD->getModule() == TargetMod)

lib/Serialization/ModuleFile.cpp

@@ -499,6 +499,10 @@ void ModuleFile::getImportDecls(SmallVectorImpl<Decl *> &Results) {
       if (Dep.isExported())
         ID->getAttrs().add(
             new (Ctx) ExportedAttr(/*IsImplicit=*/false));
+      if (Dep.isImplementationOnly())
+        ID->getAttrs().add(
+            new (Ctx) ImplementationOnlyAttr(/*IsImplicit=*/false));
+
       ImportDecls.push_back(ID);
     }
     Bits.ComputedImportDecls = true;

test/Serialization/Recovery/implementation-only-override.swift

@@ -4,7 +4,7 @@
 // RUN: %FileCheck %s < %t/Library.txt
 
 // CHECK: import FooKit
-// CHECK: import FooKit_SECRET
+// CHECK-NOT: import FooKit_SECRET
 
 import FooKit
 @_implementationOnly import FooKit_SECRET