Tests/CXXModules: enhance `internal-partitions` test

This test previously did not *require* that the internal partition be
specified as a transitive usage because nothing from it was exposed.
Plumb through usages such that the internal partitions are required.

Tests/RunCMake/CXXModules/examples/internal-partitions/importable.cxx

@@ -3,6 +3,12 @@ import :internal_partition;
 
 #include "internal-partitions_export.h"
 
+export struct module_struct
+{
+private:
+  partition_struct p;
+};
+
 export INTERNAL_PARTITIONS_EXPORT int from_import()
 {
   return from_partition();

Tests/RunCMake/CXXModules/examples/internal-partitions/internal.cxx

@@ -1,5 +1,10 @@
 export module internal;
 
+export struct internal_struct
+{
+  int x;
+};
+
 export int from_internal()
 {
   return 0;

Tests/RunCMake/CXXModules/examples/internal-partitions/main.cxx

@@ -2,5 +2,5 @@ import importable;
 
 int main(int argc, char* argv[])
 {
-  return from_import();
+  return from_import() * sizeof(module_struct);
 }

Tests/RunCMake/CXXModules/examples/internal-partitions/partition.cxx

@@ -1,6 +1,11 @@
 module importable:internal_partition;
 import internal;
 
+struct partition_struct
+{
+  internal_struct i;
+};
+
 int from_partition()
 {
   return from_internal();