Remove dangling initializers in GlobalDCE

GlobalDCE deletes global vars and updates their initializers to nullptr
while leaving underlying constants to be cleaned up later by its uses.
The clean up may never happen, fix this by forcing it every time it's
safe to destroy constants.

Final patch by Rafael Espindola
http://reviews.llvm.org/D4931

<rdar://problem/17523868>

llvm-svn: 216390