[GlobalMerge] Fix GlobalMerge on bss external global variables.

Summary:
Global variables that are external and zero initialized are
supposed to be merged with global variables in the bss section
rather than the data section.

Reviewers: efriedma, rengolin, t.p.northover, javed.absar, asl, john.brawn, pcc

Reviewed By: efriedma

Subscribers: dmgreen, llvm-commits

Differential Revision: https://reviews.llvm.org/D51379

llvm-svn: 341008