Parse DW_OP_GNU_parameter_ref, add plumbing for evaluating it.

[khuey]

DW_OP_GNU_parameter_ref is a DWARF extension that allows finding the value of parameters or variables that were optimized out of a function but may vary at the caller. See https://gcc.gnu.org/ml/gcc-patches/2011-06/msg00649.html for an example. There, the optimizer will remove both y and z from the generated assembly. Because z is constant, the debug info can note it has a value of 3. But y varies, so a DW_OP_GNU_parameter_ref is emitted. The Gimli consumer is responsible for determining the caller, finding the right DW_TAG_GNU_call_site, and looking up the DW_TAG_GNU_call_parameter's value to continue with the evaluation.

The parsing semantics of the operand are the same as DW_OP_call4s.

[coveralls]

Coverage decreased (-0.06%) to 92.977% when pulling 73daf34 on khuey:downstream into f63bac1 on gimli-rs:master.

[fitzgen]

r? @tromey

[philipc]

Note that this PR needs updating due to changed parse_u32 and Operation::parse signatures, but otherwise LGTM.

[khuey]

Rebased.

[coveralls]

Coverage decreased (-0.2%) to 93.211% when pulling 48ada86 on khuey:downstream into cf3491a on gimli-rs:master.

[philipc]

Thanks!