[C++20] [Modules] Don't perform ODR checks in GMF

Close llvm#79240.

See the linked issue for details. Given the frequency of issue reporting
about false positive ODR checks (I received private issue reports too),
I'd like to backport this to 18.x too.

clang/docs/ReleaseNotes.rst

@@ -251,6 +251,11 @@ C++20 Feature Support
 - User defined constructors are allowed for copy-list-initialization with CTAD.
   (#GH62925).
 
+- Clang won't perform ODR checks for decls in the global module fragment any
+  more to ease the implementation and improve the user's using experience.
+  This follows the MSVC's behavior.
+  (`#79240 <https://github.com/llvm/llvm-project/issues/79240>`_).
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -532,7 +532,9 @@ void ASTDeclWriter::VisitEnumDecl(EnumDecl *D) {
   EnumDeclBits.addBit(D->isFixed());
   Record.push_back(EnumDeclBits);
 
-  Record.push_back(D->getODRHash());
+  // We only perform ODR checks for decls not in GMF.
+  if (!isFromExplicitGMF(D))
+    Record.push_back(D->getODRHash());
 
   if (MemberSpecializationInfo *MemberInfo = D->getMemberSpecializationInfo()) {
     Record.AddDeclRef(MemberInfo->getInstantiatedFrom());
@@ -549,7 +551,7 @@ void ASTDeclWriter::VisitEnumDecl(EnumDecl *D) {
       !D->isTopLevelDeclInObjCContainer() &&
       !CXXRecordDecl::classofKind(D->getKind()) &&
       !D->getIntegerTypeSourceInfo() && !D->getMemberSpecializationInfo() &&
-      !needsAnonymousDeclarationNumber(D) &&
+      !needsAnonymousDeclarationNumber(D) && !isFromExplicitGMF(D) &&
       D->getDeclName().getNameKind() == DeclarationName::Identifier)
     AbbrevToUse = Writer.getDeclEnumAbbrev();
 
@@ -740,7 +742,9 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
   if (D->isExplicitlyDefaulted())
     Record.AddSourceLocation(D->getDefaultLoc());
 
-  Record.push_back(D->getODRHash());
+  // We only perform ODR checks for decls not in GMF.
+  if (!isFromExplicitGMF(D))
+    Record.push_back(D->getODRHash());
 
   if (D->isDefaulted() || D->isDeletedAsWritten()) {
     if (auto *FDI = D->getDefalutedOrDeletedInfo()) {
@@ -1552,7 +1556,8 @@ void ASTDeclWriter::VisitCXXMethodDecl(CXXMethodDecl *D) {
       D->getFirstDecl() == D->getMostRecentDecl() && !D->isInvalidDecl() &&
       !D->hasAttrs() && !D->isTopLevelDeclInObjCContainer() &&
       D->getDeclName().getNameKind() == DeclarationName::Identifier &&
-      !D->hasExtInfo() && !D->isExplicitlyDefaulted()) {
+      !isFromExplicitGMF(D) && !D->hasExtInfo() &&
+      !D->isExplicitlyDefaulted()) {
     if (D->getTemplatedKind() == FunctionDecl::TK_NonTemplate ||
         D->getTemplatedKind() == FunctionDecl::TK_FunctionTemplate ||
         D->getTemplatedKind() == FunctionDecl::TK_MemberSpecialization ||