[C++20][Modules] Rework testcase to use split file [NFC].

[iains]

Summary

This switches the testcase committed for initial C++20 modules import tracking to
use split-file rather than preprocessor directives.

[iains]

probably qualifies as trivial/obvious but in case anyone has any comments.

[ChuanqiXu]

LGTM.

clang/test/Modules/cxx20-import-diagnostics-a.cpp

 // RUN: rm -rf %t
 // RUN: mkdir -p %t
+// RUN: split-file %s %t
 
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface -D TU=0 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/import-diags-tu1.cpp \
 // RUN:  -o %t/B.pcm
 
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface -D TU=1 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/import-diags-tu2.cpp \
 // RUN:  -o %t/C.pcm
 
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface -D TU=2 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/import-diags-tu3.cpp \
 // RUN:  -fmodule-file=%t/B.pcm -fmodule-file=%t/C.pcm -o %t/AOK1.pcm
 
-// RUN: %clang_cc1 -std=c++20 -S -D TU=3 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -S %t/import-diags-tu4.cpp \
 // RUN:  -fmodule-file=%t/AOK1.pcm -o %t/tu_3.s -verify
 
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface -D TU=4 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/import-diags-tu5.cpp \
 // RUN:  -fmodule-file=%t/B.pcm -fmodule-file=%t/C.pcm -o %t/BC.pcm -verify
 
-// RUN: %clang_cc1 -std=c++20 -S -D TU=5 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -S %t/import-diags-tu6.cpp \
 // RUN:  -fmodule-file=%t/B.pcm -fmodule-file=%t/C.pcm -o %t/tu_5.s -verify
 
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface -D TU=6 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/import-diags-tu7.cpp \
 // RUN:  -fmodule-file=%t/B.pcm -o %t/D.pcm -verify
 
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface -D TU=7 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/import-diags-tu8.cpp \
 // RUN:  -fmodule-file=%t/B.pcm -o %t/D.pcm -verify
 
-// RUN: %clang_cc1 -std=c++20 -S -D TU=8 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -S %t/import-diags-tu9.cpp \
 // RUN:  -fmodule-file=%t/B.pcm -o %t/tu_8.s -verify
 
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface -D TU=9 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/import-diags-tu10.cpp \
 // RUN:  -o %t/B.pcm -verify
 
-// RUN: %clang_cc1 -std=c++20 -emit-obj -D TU=10 -x c++ %s \
+// RUN: %clang_cc1 -std=c++20 -emit-obj %t/import-diags-tu11.cpp \
 // RUN:  -fmodule-file=%t/C.pcm  -o %t/impl.o
 
 // Test diagnostics for incorrect module import sequences.
 
-#if TU == 0
+//--- import-diags-tu1.cpp
 
 export module B;
 
 int foo ();
 
 // expected-no-diagnostics
 
-#elif TU == 1
+//--- import-diags-tu2.cpp
 
 export module C;
 
 int bar ();
 
 // expected-no-diagnostics
 
-#elif TU == 2
+//--- import-diags-tu3.cpp
 
 export module AOK1;
 
 import B;
 export import C;
 
 export int theAnswer ();
 
 // expected-no-diagnostics
 
-#elif TU == 3
+//--- import-diags-tu4.cpp
 
 module;
 
 module AOK1;
 
 export import C; // expected-error {{export declaration can only be used within a module interface unit}}
 
 int theAnswer () { return 42; }
 
-#elif TU == 4
+//--- import-diags-tu5.cpp
 
 export module BC;
 
 export import B;
 
 int foo () { return 10; }
 
 import C; // expected-error {{imports must immediately follow the module declaration}}
 
-#elif TU == 5
+//--- import-diags-tu6.cpp
 
 module B; // implicitly imports B.
 
 int foo () { return 10; }
 
 import C; // expected-error {{imports must immediately follow the module declaration}}
 
-#elif TU == 6
+//--- import-diags-tu7.cpp
 
 module;
 // We can only have preprocessor commands here, which could include an include
 // translated header unit.  However those are identified specifically by the
 // preprocessor; non-preprocessed user code should not contain an import here.
 import B; // expected-error {{module imports cannot be in the global module fragment}}
 
 export module D;
 
 int delta ();
 
-#elif TU == 7
+//--- import-diags-tu8.cpp
 
 export module D;
 
 int delta ();
 
 module :private;
 
 import B; // expected-error {{module imports cannot be in the private module fragment}}
 
-#elif TU == 8
+//--- import-diags-tu9.cpp
 
 module B;
 
 import B; // expected-error {{import of module 'B' appears within same top-level module 'B'}}
 
-#elif TU == 9
+//--- import-diags-tu10.cpp
 
 export module B;
 
 import B; // expected-error {{import of module 'B' appears within same top-level module 'B'}}
 
-#elif TU == 10
+//--- import-diags-tu11.cpp
 
 int x;
 
 import C;
 
 int baz() { return 6174; }
 
 // expected-no-diagnostics
-
-#else
-#error "no MODE set"
-#endif