[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/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()) {
@@ -1558,7 +1562,8 @@ 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 ||