[NFC] Don't define static function in header (UninitializedObject.h)

Summary:
See also http://lists.llvm.org/pipermail/cfe-users/2016-January/000854.html for
the reasons why it's bad.

Reviewers: Szelethus, erichkeane

Reviewed By: Szelethus

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50963

llvm-svn: 340174