[NFC] [C++20] [Modules] Add a test to show the ability to skip bodies for import first and include later

For the common pattern:

```C++
module;
export module a;
...
```

```C++
// a.cpp
import a;
```

In this case, we're already able to skip parsing the body of some
declarations in a.cpp. Add a test to show the ability.

clang/test/Modules/skip-body.cppm

@@ -0,0 +1,63 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-reduced-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/a.cpp -fmodule-file=a=%t/a.pcm -ast-dump | FileCheck %s
+
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/a.cpp -fmodule-file=a=%t/a.pcm -ast-dump | FileCheck %s
+
+//--- a.h
+namespace a {
+class A {
+public:
+    int aaaa;
+
+    int get() {
+        return aaaa;
+    }
+};
+
+
+template <class T>
+class B {
+public:
+    B(T t): t(t) {}
+    T t;
+};
+
+using BI = B<int>;
+
+inline int get(A a, BI b) {
+    return a.get() + b.t;
+}
+
+}
+
+//--- a.cppm
+module;
+#include "a.h"
+
+export module a;
+
+namespace a {
+    export using ::a::A;
+    export using ::a::get;
+    export using ::a::BI;
+}
+
+//--- a.cpp
+import a;
+#include "a.h"
+
+int test() {
+    a::A aa;
+    a::BI bb(43);
+    return get(aa, bb);
+}
+
+// CHECK-NOT: DefinitionData
+// CHECK: FunctionDecl {{.*}} get 'int (A, BI)' {{.*}}
+// CHECK-NOT: CompoundStmt
+// CHECK: FunctionDecl {{.*}} test {{.*}}