Pointer analysis: Replace uses of namespacet::follow

[tautschnig]

This is deprecated. Use suitable variants of follow_tag instead.

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• 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).
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[codecov]

Codecov Report

Attention: Patch coverage is 83.78378% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 79.64%. Comparing base (facb34d) to head (7241dd9).

❗ Current head 7241dd9 differs from pull request most recent head 0332e47. Consider uploading reports for the commit 0332e47 to get more accurate results

Files	Patch %	Lines
src/pointer-analysis/value_set.cpp	82.05%	7 Missing ⚠️
src/pointer-analysis/value_set_fi.cpp	80.00%	3 Missing ⚠️
src/pointer-analysis/value_set_analysis_fi.cpp	81.81%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8231      +/-   ##
===========================================
+ Coverage    77.96%   79.64%   +1.67%     
===========================================
  Files         1720     1684      -36     
  Lines       188982   195679    +6697     
  Branches     18391        0   -18391     
===========================================
+ Hits        147344   155852    +8508     
+ Misses       41638    39827    -1811     

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