[NFC][C++][Modules] Refine test/CXX/basic.link/p3.cpp with split-file (#147945)

Refine basic.link.p3 tests with split-file.

---------

Signed-off-by: Wang, Yihan <yronglin777@gmail.com>

Author: yronglin

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

@@ -1,35 +1,18 @@
-// RUN: %clang_cc1 -std=c++2a -verify %s
-// RUN: %clang_cc1 -std=c++2a -verify %s -DIMPORT_ERROR=1
-// RUN: %clang_cc1 -std=c++2a -verify %s -DIMPORT_ERROR=2
+// RUN: rm -rf %t
+// RUN: split-file %s %t
 
+// RUN: %clang_cc1 -std=c++20 -verify %t/M.cpp
+// RUN: %clang_cc1 -std=c++20 -verify %t/ImportError1.cpp
+// RUN: %clang_cc1 -std=c++20 -verify %t/ImportError2.cpp
+
+//--- M.cpp
 module;
 
-#if IMPORT_ERROR != 2
 struct import { struct inner {}; };
-#endif
 struct module { struct inner {}; };
-
 constexpr int n = 123;
 
 export module m; // #1
-
-// Import errors are fatal, so we test them in isolation.
-#if IMPORT_ERROR == 1
-import x = {}; // expected-error {{expected ';' after module name}}
-               // expected-error@-1 {{module 'x' not found}}
-
-#elif IMPORT_ERROR == 2
-struct X;
-template<int> struct import;
-template<> struct import<n> {
-  static X y;
-};
-
-// This is not valid because the 'import <n>' is a pp-import, even though it
-// grammatically can't possibly be an import declaration.
-struct X {} import<n>::y; // expected-error {{'n' file not found}}
-
-#else
 module y = {}; // expected-error {{multiple module declarations}} expected-error 2{{}}
 // expected-note@#1 {{previous module declaration}}
 
@@ -51,4 +34,36 @@ template<typename T> module module_var_template;
 
 // This is a variable named 'import' that shadows the type 'import' above.
 struct X {} import;
-#endif
+
+//--- ImportError1.cpp
+module;
+
+struct import { struct inner {}; };
+struct module { struct inner {}; };
+
+constexpr int n = 123;
+
+export module m; // #1
+
+import x = {}; // expected-error {{expected ';' after module name}}
+               // expected-error@-1 {{module 'x' not found}}
+
+//--- ImportError2.cpp
+module;
+
+struct module { struct inner {}; };
+
+constexpr int n = 123;
+
+export module m; // #1
+
+struct X;
+template<int> struct import;
+template<> struct import<n> {
+  static X y;
+};
+
+// This is not valid because the 'import <n>' is a pp-import, even though it
+// grammatically can't possibly be an import declaration.
+struct X {} import<n>::y; // expected-error {{'n' file not found}}
+