Teach ValueMapper to use ODR uniqued types when available

Summary:
This is exposed during ThinLTO compilation, when we import an alias by
creating a clone of the aliasee. Without this fix the debug type is
unnecessarily cloned and we get a duplicate, undoing the uniquing.

Fixes PR36089.

Reviewers: mehdi_amini, pcc

Subscribers: eraman, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D41669

llvm-svn: 323813