[NFCI] [C++20] [Modules] Relax the case for duplicated declaration in multiple module units for explicit specialization

Relax the case for duplicated declaration in multiple module units for
explicit specialization and refactor the implementation of
checkMultipleDefinitionInNamedModules a little bit.

This is intended to not affect any end users since it only relaxes the
condition to emit an error.

Author: ChuanqiXu9

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -3692,12 +3692,6 @@ static void inheritDefaultTemplateArguments(ASTContext &Context,
 //    the program is ill-formed;
 static void checkMultipleDefinitionInNamedModules(ASTReader &Reader, Decl *D,
                                                   Decl *Previous) {
-  Module *M = Previous->getOwningModule();
-
-  // We only care about the case in named modules.
-  if (!M || !M->isNamedModule())
-    return;
-
   // If it is previous implcitly introduced, it is not meaningful to
   // diagnose it.
   if (Previous->isImplicit())
@@ -3714,16 +3708,21 @@ static void checkMultipleDefinitionInNamedModules(ASTReader &Reader, Decl *D,
   // FIXME: Maybe this shows the implicit instantiations may have incorrect
   // module owner ships. But given we've finished the compilation of a module,
   // how can we add new entities to that module?
-  if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(Previous);
-      VTSD && !VTSD->isExplicitSpecialization())
+  if (isa<VarTemplateSpecializationDecl>(Previous))
     return;
-  if (auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(Previous);
-      CTSD && !CTSD->isExplicitSpecialization())
+  if (isa<ClassTemplateSpecializationDecl>(Previous))
+    return;
+  if (auto *Func = dyn_cast<FunctionDecl>(Previous);
+      Func && Func->getTemplateSpecializationInfo())
+    return;
+
+  Module *M = Previous->getOwningModule();
+  if (!M)
+    return;
+
+  // We only forbids merging decls within named modules.
+  if (!M->isNamedModule())
     return;
-  if (auto *Func = dyn_cast<FunctionDecl>(Previous))
-    if (auto *FTSI = Func->getTemplateSpecializationInfo();
-        FTSI && !FTSI->isExplicitSpecialization())
-      return;
 
   // It is fine if they are in the same module.
   if (Reader.getContext().isInSameModule(M, D->getOwningModule()))