[C++20] [Modules] handling selectAny attribute for vardecl

Close llvm#127963

The root cause of the problem seems to be that we didn't realize it
simply.

(cherry picked from commit 24c06a1)

Author: ChuanqiXu9

clang/lib/Sema/SemaDecl.cpp

@@ -4803,7 +4803,8 @@ bool Sema::checkVarDeclRedefinition(VarDecl *Old, VarDecl *New) {
       (New->getFormalLinkage() == Linkage::Internal || New->isInline() ||
        isa<VarTemplateSpecializationDecl>(New) ||
        New->getDescribedVarTemplate() || New->getNumTemplateParameterLists() ||
-       New->getDeclContext()->isDependentContext())) {
+       New->getDeclContext()->isDependentContext() ||
+       New->hasAttr<SelectAnyAttr>())) {
     // The previous definition is hidden, and multiple definitions are
     // permitted (in separate TUs). Demote this to a declaration.
     New->demoteThisDefinitionToDeclaration();

clang/test/Modules/pr127943.cppm

@@ -0,0 +1,31 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/repro.cppm -fdeclspec -emit-module-interface -o %t/repro.pcm
+// RUN: %clang_cc1 -std=c++20 %t/source.cpp -fdeclspec -fsyntax-only -verify -fprebuilt-module-path=%t
+
+//--- repro_decl.hpp
+#pragma once
+
+extern "C"
+{
+    __declspec(selectany) int foo = 0;
+}
+
+//--- repro.cppm
+module;
+#include "repro_decl.hpp"
+
+export module repro;
+
+export inline int func()
+{
+    return foo;
+}
+
+//--- source.cpp
+// expected-no-diagnostics
+import repro;
+
+#include "repro_decl.hpp"