nondet-volatile: fix handling of enum types

[tautschnig]

namespacet::follow does not follow enum tags. Use follow_tag instead
to avoid unbounded recursion when using nondet-volatile on programs that
use enums.

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (1939544) 79.66% compared to head (4465a38) 79.66%.

Files	Patch %	Lines
src/goto-instrument/nondet_volatile.cpp	66.66%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8203   +/-   ##
========================================
  Coverage    79.66%   79.66%           
========================================
  Files         1682     1682           
  Lines       195378   195380    +2     
========================================
+ Hits        155642   155646    +4     
+ Misses       39736    39734    -2     

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