[C++20] [Modules] Mark exported all declarations as used

Close https://github.com/llvm/llvm-project/issues/85122

As the title suggested, it looks pretty sensible.

clang/lib/Sema/SemaModule.cpp

@@ -1003,6 +1003,10 @@ Decl *Sema::ActOnFinishExportDecl(Scope *S, Decl *D, SourceLocation RBraceLoc) {
     }
   }
 
+  // Anything exported from a module should never be considered unused.
+  for (auto *Exported : ED->decls())
+    Exported->markUsed(getASTContext());
+
   return D;
 }
 

clang/test/Modules/pr85122.cppm

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -std=c++20 %s -Wall -fsyntax-only -verify
+
+// expected-no-diagnostics
+export module a;
+
+export constexpr auto a = []{};