Enable ODR uniquing of DITypes for ThinLTO backends

Summary:
This is a follow-on to apply Duncan's new DIType ODR uniquing from
r266549 and r266713 in more places.

When invoking ThinLTO backend compiles via clang (for a distributed
build), invoke enableDebugTypeODRUniquing() before parsing the module.

Reviewers: dexonsmith, joker.eph

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D19264

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@266852 91177308-0d34-0410-b5e6-96231b3b80d8