IR: Call dropAllReferences from GlobalVariable's destructor.

We were previously failing to do this and as a result failing to drop
attached metadata.

Not sure if there's a good way to test this. An in-progress patch exposed this
problem by allocating a GlobalVariable at the same address as a previously
allocated GlobalVariable.

Differential Revision: http://reviews.llvm.org/D21109

llvm-svn: 272077