Pass the .ll files to llvm-link directly. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222681 91177308-0d34-0410-b5e6-96231b3b80d8
Add and use Type::subtypes. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222682 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the unused FindUsedTypes pass.

It was dead since r134829.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222684 91177308-0d34-0410-b5e6-96231b3b80d8