[Modules] Add test for merging of template member parent

This is a reduced test case originally meant to be addressed by
https://reviews.llvm.org/D137787. It was recently fixed by commit
61c7a914 ("Commit to a primary definition for a class when we load
its first member."), noting the difficulty to come up with a reduced
test case. This setup with four modules seems to fail consistently
before the fix mentioned above with an assertion in CGExprCXX.cpp,
CodeGenFunction::EmitCXXDestructorCall():
Assertion `ThisTy->getAsCXXRecordDecl() == DtorDecl->getParent() &&
"Pointer/Object mixup"' failed.

Differential Revision: https://reviews.llvm.org/D156806