Revert r342019, "Track definition merging on the canonical declaration
even when [...]"

Further testing has revealed that this causes build breaks during
explicit module compilations.

llvm-svn: 342020
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index ac3b737..b60e5f1 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3780,15 +3780,22 @@
 /// visible.
 void ASTReader::mergeDefinitionVisibility(NamedDecl *Def,
                                           NamedDecl *MergedDef) {
+  // FIXME: This doesn't correctly handle the case where MergedDef is visible
+  // in modules other than its owning module. We should instead give the
+  // ASTContext a list of merged definitions for Def.
   if (Def->isHidden()) {
     // If MergedDef is visible or becomes visible, make the definition visible.
     if (!MergedDef->isHidden())
       Def->setVisibleDespiteOwningModule();
-    else {
+    else if (getContext().getLangOpts().ModulesLocalVisibility) {
       getContext().mergeDefinitionIntoModule(
           Def, MergedDef->getImportedOwningModule(),
           /*NotifyListeners*/ false);
       PendingMergedDefinitionsToDeduplicate.insert(Def);
+    } else {
+      auto SubmoduleID = MergedDef->getOwningModuleID();
+      assert(SubmoduleID && "hidden definition in no module");
+      HiddenNamesMap[getSubmodule(SubmoduleID)].push_back(Def);
     }
   }
 }