[C++20] [Modules] Diagnose if import statement lakcs a semicolon

Close #121066

Now we will diagnose that the import statement lacks a semicolon as
expected. Note that the original "not found" diagnose message remains.
I meant to remove that, but the test shows it might be more complex
process (other unexpected diagnose shows up). Given the importance of
the issue, I chose to not dig deeper.

Author: ChuanqiXu9

clang/lib/Parse/Parser.cpp

@@ -2654,10 +2654,10 @@ Decl *Parser::ParseModuleImport(SourceLocation AtLoc,
       SeenError = false;
     break;
   }
-  if (SeenError) {
-    ExpectAndConsumeSemi(diag::err_module_expected_semi);
+  ExpectAndConsumeSemi(diag::err_module_expected_semi);
+
+  if (SeenError)
     return nullptr;
-  }
 
   DeclResult Import;
   if (HeaderUnit)
@@ -2666,7 +2666,6 @@ Decl *Parser::ParseModuleImport(SourceLocation AtLoc,
   else if (!Path.empty())
     Import = Actions.ActOnModuleImport(StartLoc, ExportLoc, ImportLoc, Path,
                                        IsPartition);
-  ExpectAndConsumeSemi(diag::err_module_expected_semi);
   if (Import.isInvalid())
     return nullptr;
 

clang/test/CXX/basic/basic.link/p3.cpp

@@ -15,7 +15,8 @@ export module m; // #1
 
 // Import errors are fatal, so we test them in isolation.
 #if IMPORT_ERROR == 1
-import x = {}; // expected-error {{module 'x' not found}}
+import x = {}; // expected-error {{expected ';' after module name}}
+               // expected-error@-1 {{module 'x' not found}}
 
 #elif IMPORT_ERROR == 2
 struct X;

clang/test/Modules/pr121066.cpp

@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only %s -verify 
+
+import mod // expected-error {{expected ';' after module name}}
+           // expected-error@-1 {{module 'mod' not found}}