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

llvm-svn: 266852