Revert "[modules] Fix miscompilation when using two RecordDecl definitions with the same name."

This reverts commit 9454716.

The patch being reverted results in swift-corelibs-foundation failing to
build due to being unable to find `Glibc.pthread_mutex_t`. The
declaration is defined in the module, but is also imported via
CDispatch. This patch marks any complete redeclarations of a given
declaration incomplete. Swift can't import incomplete declarations, so
it is ignored in `ClangImporter::isVisibleFromModule`. We will need some
mechanism of detecting redeclarations while not marking complete
declarations as being incomplete.

Author: etcwilde

clang/include/clang/Serialization/ASTReader.h

@@ -1155,10 +1155,6 @@ class ASTReader
   /// definitions. Only populated when using modules in C++.
   llvm::DenseMap<EnumDecl *, EnumDecl *> EnumDefinitions;
 
-  /// A mapping from canonical declarations of records to their canonical
-  /// definitions. Doesn't cover CXXRecordDecl.
-  llvm::DenseMap<RecordDecl *, RecordDecl *> RecordDefinitions;
-
   /// When reading a Stmt tree, Stmt operands are placed in this stack.
   SmallVector<Stmt *, 16> StmtStack;
 

clang/lib/Serialization/ASTCommon.cpp

@@ -477,7 +477,7 @@ bool serialization::needsAnonymousDeclarationNumber(const NamedDecl *D) {
   // Otherwise, we only care about anonymous class members / block-scope decls.
   // FIXME: We need to handle lambdas and blocks within inline / templated
   // variables too.
-  if (D->getDeclName() || !isa<RecordDecl>(D->getLexicalDeclContext()))
+  if (D->getDeclName() || !isa<CXXRecordDecl>(D->getLexicalDeclContext()))
     return false;
   return isa<TagDecl>(D) || isa<FieldDecl>(D);
 }

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -332,7 +332,7 @@ namespace clang {
     RedeclarableResult VisitTagDecl(TagDecl *TD);
     void VisitEnumDecl(EnumDecl *ED);
     RedeclarableResult VisitRecordDeclImpl(RecordDecl *RD);
-    void VisitRecordDecl(RecordDecl *RD);
+    void VisitRecordDecl(RecordDecl *RD) { VisitRecordDeclImpl(RD); }
     RedeclarableResult VisitCXXRecordDeclImpl(CXXRecordDecl *D);
     void VisitCXXRecordDecl(CXXRecordDecl *D) { VisitCXXRecordDeclImpl(D); }
     RedeclarableResult VisitClassTemplateSpecializationDeclImpl(
@@ -808,34 +808,6 @@ ASTDeclReader::VisitRecordDeclImpl(RecordDecl *RD) {
   return Redecl;
 }
 
-void ASTDeclReader::VisitRecordDecl(RecordDecl *RD) {
-  VisitRecordDeclImpl(RD);
-
-  // Maintain the invariant of a redeclaration chain containing only
-  // a single definition.
-  if (RD->isCompleteDefinition()) {
-    RecordDecl *Canon = static_cast<RecordDecl *>(RD->getCanonicalDecl());
-    RecordDecl *&OldDef = Reader.RecordDefinitions[Canon];
-    if (!OldDef) {
-      // This is the first time we've seen an imported definition. Look for a
-      // local definition before deciding that we are the first definition.
-      for (auto *D : merged_redecls(Canon)) {
-        if (!D->isFromASTFile() && D->isCompleteDefinition()) {
-          OldDef = D;
-          break;
-        }
-      }
-    }
-    if (OldDef) {
-      Reader.MergedDeclContexts.insert(std::make_pair(RD, OldDef));
-      RD->setCompleteDefinition(false);
-      Reader.mergeDefinitionVisibility(OldDef, RD);
-    } else {
-      OldDef = RD;
-    }
-  }
-}
-
 void ASTDeclReader::VisitValueDecl(ValueDecl *VD) {
   VisitNamedDecl(VD);
   // For function declarations, defer reading the type in case the function has
@@ -2689,7 +2661,7 @@ static bool allowODRLikeMergeInC(NamedDecl *ND) {
   if (!ND)
     return false;
   // TODO: implement merge for other necessary decls.
-  if (isa<EnumConstantDecl, FieldDecl, IndirectFieldDecl>(ND))
+  if (isa<EnumConstantDecl>(ND))
     return true;
   return false;
 }
@@ -3361,9 +3333,6 @@ DeclContext *ASTDeclReader::getPrimaryContextForMerging(ASTReader &Reader,
     return DD->Definition;
   }
 
-  if (auto *RD = dyn_cast<RecordDecl>(DC))
-    return RD->getDefinition();
-
   if (auto *ED = dyn_cast<EnumDecl>(DC))
     return ED->getASTContext().getLangOpts().CPlusPlus? ED->getDefinition()
                                                       : nullptr;
@@ -3450,9 +3419,6 @@ ASTDeclReader::getPrimaryDCForAnonymousDecl(DeclContext *LexicalDC) {
     if (auto *MD = dyn_cast<ObjCMethodDecl>(D))
       if (MD->isThisDeclarationADefinition())
         return MD;
-    if (auto *RD = dyn_cast<RecordDecl>(D))
-      if (RD->isThisDeclarationADefinition())
-        return RD;
   }
 
   // No merged definition yet.