[clang][sema] Add missing diagnostic parameter

[tbaeder]

Summary

The test case otherwise fails an assertion in Diagnostic::getArgKind():

clang++: /home/tbaeder/code/llvm-project/clang/include/clang/Basic/Diagnostic.h:1571: clang::DiagnosticsEngine::ArgumentKind clang::Diagnostic::getArgKind(unsigned int) const: Assertion `Idx < getNumArgs() && "Argument index out of range!"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: bin/clang++ module_import.cpp -std=c++20 -fmodules -fimplicit-module-maps -I. -stdlib=libc++ -fsyntax-only
1.	module_import.cpp:1:20: current parser token ';'

...

[aaron.ballman]

clang/test/Modules/cxx20-export-import.cpp

1–2

Also, does this test require -fmodules-cache-path=%t or -fimplicit-module-maps?

[tbaeder]

clang/test/Modules/cxx20-export-import.cpp

1–2

They are all needed (including the -I). I could only remove the -stdlib=libc++, removing any of the others makes the test fail :(

[aaron.ballman]

I think the test behavior is pretty mysterious and there's likely some other bug happening, but this is incremental progress and definitely fixes an issue, so LGTM!

clang/test/Modules/cxx20-export-import.cpp

1–2

Oh wow, that's... pretty surprising behavior.

clang/lib/Sema/SemaModule.cpp

if (!ModuleScopes.empty() && ModuleScopes.back().ModuleInterface) {
     if (ExportLoc.isValid() || getEnclosingExportDecl(Import))
       getCurrentModule()->Exports.emplace_back(Mod, false);
     else
       getCurrentModule()->Imports.insert(Mod);
   } else if (ExportLoc.isValid()) {
     // [module.interface]p1:
     // An export-declaration shall inhabit a namespace scope and appear in the
     // purview of a module interface unit.
-    Diag(ExportLoc, diag::err_export_not_in_module_interface);
+    Diag(ExportLoc, diag::err_export_not_in_module_interface) << 0;
   }
 
   return Import;
 }
 
 void Sema::ActOnModuleInclude(SourceLocation DirectiveLoc, Module *Mod) {
   checkModuleImportContext(*this, Mod, DirectiveLoc, CurContext, true);
   BuildModuleInclude(DirectiveLoc, Mod);

clang/test/Modules/cxx20-export-import.cpp

+
+// RUN: %clang_cc1 -std=c++20 -fmodules -fmodules-cache-path=%t -fimplicit-module-maps -I%S/Inputs -stdlib=libc++ -verify %s
+export import dummy; // expected-error {{export declaration can only be used within a module interface unit after the module declaration}}