Fix a bug that prevents global variables from having a DW_OP_deref.

Fix a bug that prevents global variables from having a DW_OP_deref.

For local variables the first DW_OP_deref is consumed by turning the
location kind into a memeory location, but that only makes sense for
values that are in a register to begin with, which cannot happen for
global variables that are attached to a symbol.

rdar://problem/39741860

This reapplies r330970 after fixing an uncovered bug in r331086 and
working around the situation caused by it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331090 91177308-0d34-0410-b5e6-96231b3b80d8