[ThinLTO] Add a llvm-lto2 test to check that ODR type uniquing is enabled (NFC)

This adds a test for r279532, thanks David Li for noticing :)

Recommit r279545 after committing first a dependent patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279551 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed