Rename follow_struct_or_union_tag to follow_tag

[tautschnig]

With the new struct_or_union_tag_typet the API is entirely consistent with the other follow_tag variants. Invoking
follow_struct_or_union_tag(to_struct_or_union_tag_type(t)) just made for awkwardly looking code.

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• 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 79.64%. Comparing base (e7b0557) to head (bb8d706).

Files	Patch %	Lines
src/util/namespace.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8251   +/-   ##
========================================
  Coverage    79.64%   79.64%           
========================================
  Files         1684     1684           
  Lines       195670   195670           
========================================
  Hits        155841   155841           
  Misses       39829    39829           

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