[ModuleUtils] NFC: Add unit tests for appendToUsedList

This patch adds initial unit tests for appendToUsedList
in the ModuleUtils. It specifically tests changes from
https://reviews.llvm.org/D101363 which intent to allow
insertion of globals in non-zero address spaces into the
llvm used lists.

Reviewed by: dblaikie

Differential Revision: https://reviews.llvm.org/D101746