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[SystemZ] Fix handling of pass-by-pointer arguments

I'd misunderstood getIndirect() to mean that the argument should be passed
as a pointer at the ABI level, with the ByVal argument choosing caller-copy
semantics over no-caller-copy (callee-copy-on-write) semantics.  But
getIndirect(x) actually means that x is passed by pointer at the IR
level but (at least on all other targets I looked at) directly at the
ABI level.  getIndirect(x, false) selects a pointer to a caller-made
copy, which is what SystemZ was aiming for.

This fixes a miscompilation of c-index-test.  Structure arguments were being
passed by pointer, but no copy was being made, so a write in the callee
stomped over a caller's local variable.