Make __has_unique_object_representations reject empty union types.

Summary:
Clang incorrectly reports empty unions as having a unique object representation. However, this is not correct since `sizeof(EmptyUnion) == 1` AKA it has 8 bits of padding. Therefore it should be treated the same as an empty struct and report `false`.

@erichkeane also suggested this fix should be merged into the 6.0 release branch, so the initial release of `__has_unique_object_representations` is as bug-free as possible. 

Reviewers: erichkeane, rsmith, aaron.ballman, majnemer

Reviewed By: erichkeane

Subscribers: cfe-commits, erichkeane

Differential Revision: https://reviews.llvm.org/D42863

llvm-svn: 324134