[C++20] [Modules] Handle import decl before module declaration without being in GMF

Close #67627

In a module unit, all the declaration before the modoule declaration
should live in the GMF.

Author: ChuanqiXu9

clang/lib/Parse/Parser.cpp

@@ -2564,6 +2564,10 @@ Decl *Parser::ParseModuleImport(SourceLocation AtLoc,
     SeenError = false;
     break;
   case Sema::ModuleImportState::FirstDecl:
+    // If we found an import decl as the first declaration, we must be not in
+    // a C++20 module unit or we are in an invalid state.
+    ImportState = Sema::ModuleImportState::NotACXX20Module;
+    [[fallthrough]];
   case Sema::ModuleImportState::NotACXX20Module:
     // We can only import a partition within a module purview.
     if (IsPartition)

clang/test/Modules/pr67627.cppm

@@ -0,0 +1,13 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/A.cppm -emit-module-interface -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 %t/B.cppm -fmodule-file=A=%t/A.pcm -fsyntax-only -verify
+
+//--- A.cppm
+export module A;
+
+//--- B.cppm
+import A; // expected-note {{add 'module;' to the start of the file to introduce a global module fragment}}
+export module B; // expected-error {{module declaration must occur at the start of the translation unit}}