Implementation-only import checking for types used in decls

Based on the existing access checker for types used in decls. There's
a common skeleton here but we can't seem to get it out, so for now
add a third DeclVisitor to this file. On the plus side, checking this
alongside access is an easy way to make sure everything gets checked.

Part of rdar://problem/48991061

Author: jrose-apple

include/swift/AST/DiagnosticsSema.def

@@ -2392,6 +2392,11 @@ NOTE(enum_raw_value_incrementing_from_zero,none,
 NOTE(construct_raw_representable_from_unwrapped_value,none,
      "construct %0 from unwrapped %1 value", (Type, Type))
 
+ERROR(decl_from_implementation_only_module,none,
+      "cannot use %0 here; %1 has been imported as "
+      "'@_implementationOnly'",
+      (DeclName, Identifier))
+
 // Derived conformances
 ERROR(cannot_synthesize_init_in_extension_of_nonfinal,none,
       "implementation of %0 for non-final class cannot be automatically "

lib/AST/AccessScopeChecker.cpp

@@ -28,7 +28,7 @@ AccessScopeChecker::AccessScopeChecker(const DeclContext *useDC,
 
 bool
 AccessScopeChecker::visitDecl(const ValueDecl *VD) {
-  if (!VD || isa<GenericTypeParamDecl>(VD))
+  if (isa<GenericTypeParamDecl>(VD))
     return true;
 
   auto AS = VD->getFormalAccessScope(File, TreatUsableFromInlineAsPublic);