[NFC] [C++20] [Modules] Use Sema::isCurrentModulePurview() to simplify the codes

Author: ChuanqiXu9

clang/lib/Sema/Sema.cpp

@@ -1216,9 +1216,7 @@ void Sema::ActOnEndOfTranslationUnit() {
     // module declaration by now.
     if (getLangOpts().getCompilingModule() ==
             LangOptions::CMK_ModuleInterface &&
-        (ModuleScopes.empty() ||
-         !ModuleScopes.back().Module->isModulePurview()) &&
-        !DiagnosedMissingModuleDeclaration) {
+        !isCurrentModulePurview() && !DiagnosedMissingModuleDeclaration) {
       // FIXME: Make a better guess as to where to put the module declaration.
       Diag(getSourceManager().getLocForStartOfFile(
                getSourceManager().getMainFileID()),

clang/lib/Sema/SemaModule.cpp

@@ -207,8 +207,7 @@ Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
   // here, in order to support macro import.
 
   // Only one module-declaration is permitted per source file.
-  if (!ModuleScopes.empty() &&
-      ModuleScopes.back().Module->isModulePurview()) {
+  if (isCurrentModulePurview()) {
     Diag(ModuleLoc, diag::err_module_redeclaration);
     Diag(VisibleModules.getImportLoc(ModuleScopes.back().Module),
          diag::note_prev_module_declaration);
@@ -717,7 +716,7 @@ Decl *Sema::ActOnStartExportDecl(Scope *S, SourceLocation ExportLoc,
   //   An export-declaration shall appear only [...] in the purview of a module
   //   interface unit. An export-declaration shall not appear directly or
   //   indirectly within [...] a private-module-fragment.
-  if (ModuleScopes.empty() || !ModuleScopes.back().Module->isModulePurview()) {
+  if (!isCurrentModulePurview()) {
     Diag(ExportLoc, diag::err_export_not_in_module_interface) << 0;
     D->setInvalidDecl();
     return D;