[Modules] Don't re-generate template specialization in the importer

Close #60693.

In this issue, we can find that the importer will try to generate the
template specialization again in the importer, which is not good and
wastes time. This patch tries to address the problem.

clang/include/clang/AST/ASTContext.h

@@ -3043,7 +3043,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   }
 
   GVALinkage GetGVALinkageForFunction(const FunctionDecl *FD) const;
-  GVALinkage GetGVALinkageForVariable(const VarDecl *VD);
+  GVALinkage GetGVALinkageForVariable(const VarDecl *VD) const;
 
   /// Determines if the decl can be CodeGen'ed or deserialized from PCH
   /// lazily, only when used; this is only relevant for function or file scoped

clang/lib/AST/ASTContext.cpp

@@ -11764,7 +11764,7 @@ static GVALinkage basicGVALinkageForVariable(const ASTContext &Context,
   llvm_unreachable("Invalid Linkage!");
 }
 
-GVALinkage ASTContext::GetGVALinkageForVariable(const VarDecl *VD) {
+GVALinkage ASTContext::GetGVALinkageForVariable(const VarDecl *VD) const {
   return adjustGVALinkageForExternalDefinitionKind(*this, VD,
            adjustGVALinkageForAttributes(*this, VD,
              basicGVALinkageForVariable(*this, VD)));

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -1068,7 +1068,7 @@ void ASTDeclWriter::VisitVarDecl(VarDecl *D) {
   if (D->getStorageDuration() == SD_Static) {
     bool ModulesCodegen = false;
     if (Writer.WritingModule &&
-        !D->getDescribedVarTemplate() && !D->getMemberSpecializationInfo()) {
+        !D->getDescribedVarTemplate()) {
       // When building a C++20 module interface unit or a partition unit, a
       // strong definition in the module interface is provided by the
       // compilation of that unit, not by its users. (Inline variables are still
@@ -1077,7 +1077,7 @@ void ASTDeclWriter::VisitVarDecl(VarDecl *D) {
           (Writer.WritingModule->isInterfaceOrPartition() ||
            (D->hasAttr<DLLExportAttr>() &&
             Writer.Context->getLangOpts().BuildingPCHWithObjectFile)) &&
-          Writer.Context->GetGVALinkageForVariable(D) == GVA_StrongExternal;
+           Writer.Context->GetGVALinkageForVariable(D) >= GVA_StrongExternal;
     }
     Record.push_back(ModulesCodegen);
     if (ModulesCodegen)
@@ -2550,7 +2550,7 @@ void ASTRecordWriter::AddFunctionDefinition(const FunctionDecl *FD) {
       // compilation of that unit, not by its users. (Inline functions are still
       // emitted in module users.)
       Linkage = Writer->Context->GetGVALinkageForFunction(FD);
-      ModulesCodegen = *Linkage == GVA_StrongExternal;
+      ModulesCodegen = *Linkage >= GVA_StrongExternal;
     }
     if (Writer->Context->getLangOpts().ModulesCodegen ||
         (FD->hasAttr<DLLExportAttr>() &&

clang/test/Modules/pr59780.cppm

@@ -33,14 +33,14 @@ struct Y<int> {
 
 int Y<int>::value = 0;
 
-// CHECK-NOT: @_ZW1a1xIiE = {{.*}}available_externally{{.*}}global
-// CHECK-NOT: @_ZNW1a1YIiE5valueE = {{.*}}available_externally{{.*}}global
+// CHECK-NOT: @_ZW1a1xIiE = {{.*}}external{{.*}}global
+// CHECK-NOT: @_ZNW1a1YIiE5valueE = {{.*}}external{{.*}}global
 
 //--- use.cpp
 import a;
 int foo() {
     return x<int> + Y<int>::value;
 }
 
-// CHECK: @_ZW1a1xIiE = {{.*}}available_externally{{.*}}global
-// CHECK: @_ZNW1a1YIiE5valueE = {{.*}}available_externally{{.*}}global
+// CHECK: @_ZW1a1xIiE = {{.*}}external{{.*}}global
+// CHECK: @_ZNW1a1YIiE5valueE = {{.*}}external{{.*}}global

clang/test/Modules/pr60693.cppm

@@ -0,0 +1,54 @@
+// Address: https://github.com/llvm/llvm-project/issues/60693
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -fmodule-file=a=%t/a.pcm %t/c.cpp -S -emit-llvm -disable-llvm-passes -o - | FileCheck %t/c.cpp
+
+//--- a.cppm
+export module a;
+
+constexpr bool f() {
+	for (unsigned n = 0; n != 10; ++n) {
+	}
+	return true;
+}
+
+export template<typename>
+struct s {
+	static constexpr auto a = f();
+	static constexpr auto b = f();
+	static constexpr auto c = f();
+	static constexpr auto d = f();
+    int foo() {
+        return 43;
+    }
+    int bar() {
+        return 44;
+    }
+};
+
+template struct s<int>;
+template struct s<long>;
+
+//--- c.cpp
+import a;
+
+extern "C" int use() {
+    s<int> _;
+    return _.a + _.b + _.c + _.d;
+}
+
+extern "C" long use2() {
+    s<long> _;
+    return _.foo();
+}
+
+// CHECK: define{{.*}}@use(
+// CHECK-NOT: }
+// CHECK: ret{{.*}} 4
+
+// CHECK: declare{{.*}}@_ZNW1a1sIlE3fooEv
+// CHECK-NOT: _ZNW1a1sIlE3barEv