[llvm-reduce] ReduceGlobalVarInitializers delta pass: fix handling of globals...

[llvm-reduce] ReduceGlobalVarInitializers delta pass: fix handling of globals w/ comdat/non-external linkage

Much like with ReduceFunctionBodies delta pass,
we need to remove comdat and set linkage to external,
else verifier will complain, and our deltas are invalid.