Replace clang::Decl::isHidden() with clang::Sema::isVisible().

clang::Decl::isHidden() is misleadingly named (see the associated FIXME
comment) and doesn't do what we want when
`-fmodules-local-submodule-visibility` is turned on.

This change alone isn't unfortunately enough to make Swift work with
`-fmodules-local-submodule-visibility`. For one thing, Clang's
Objective-C component  doesn't work correctly with
`-fmodules-local-submodule-visibility`:

https://bugs.llvm.org/show_bug.cgi?id=46248

However, in the meantime, we should still clean up our use of
clang::Decl::isHidden().

Author: martinboehme

lib/ClangImporter/ClangImporter.cpp

@@ -2889,11 +2889,12 @@ void ClangModuleUnit::lookupValue(DeclName name, NLKind lookupKind,
 bool ClangImporter::Implementation::isVisibleClangEntry(
     const clang::NamedDecl *clangDecl) {
   // For a declaration, check whether the declaration is hidden.
-  if (!clangDecl->isHidden()) return true;
+  clang::Sema &clangSema = getClangSema();
+  if (clangSema.isVisible(clangDecl)) return true;
 
   // Is any redeclaration visible?
   for (auto redecl : clangDecl->redecls()) {
-    if (!cast<clang::NamedDecl>(redecl)->isHidden()) return true;
+    if (clangSema.isVisible(cast<clang::NamedDecl>(redecl))) return true;
   }
 
   return false;
@@ -3004,8 +3005,10 @@ void ClangImporter::loadExtensions(NominalTypeDecl *nominal,
     SmallVector<clang::NamedDecl *, 4> DelayedCategories;
 
     // Simply importing the categories adds them to the list of extensions.
-    for (const auto *Cat : objcClass->visible_categories()) {
-      Impl.importDeclReal(Cat, Impl.CurrentVersion);
+    for (const auto *Cat : objcClass->known_categories()) {
+      if (getClangSema().isVisible(Cat)) {
+        Impl.importDeclReal(Cat, Impl.CurrentVersion);
+      }
     }
   }
 

lib/ClangImporter/ImportDecl.cpp

@@ -7131,9 +7131,12 @@ void SwiftDeclConverter::importMirroredProtocolMembers(
           return;
 
         bool inNearbyCategory =
-            std::any_of(interfaceDecl->visible_categories_begin(),
-                        interfaceDecl->visible_categories_end(),
+            std::any_of(interfaceDecl->known_categories_begin(),
+                        interfaceDecl->known_categories_end(),
                         [=](const clang::ObjCCategoryDecl *category) -> bool {
+                          if (!Impl.getClangSema().isVisible(category)) {
+                            return false;
+                          }
                           if (category != decl) {
                             auto *categoryModule =
                                 Impl.getClangModuleForDecl(category);