[tautschnig]

smt2_convt::parse_rec had no case for c_bit_field types and returned
nil, so traces showed bit-field members as "(assignment removed)".
Parse the value using the integer type of the bit-field's width and
signedness (c_bit_field_replacement_type), and restore the bit-field
type.

This only affects counterexample traces of the SMT2 back ends, not
verification results.

Fixes #9346

Found by Jeroen Robben's specification-conformance pipeline (https://reports.robben.io/methodology/).

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