String abstraction: use tag types

[tautschnig]

String abstraction would previously place expanded struct types in
expressions. All other analyses, however, expect exclusive use of
struct_tag_types.

This problem became apparent when debugging nondet pointers.

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6477 (ba90cd3) into develop (2a9e3e2) will decrease coverage by 0.00%.
The diff coverage is 98.50%.

@@             Coverage Diff             @@
##           develop    #6477      +/-   ##
===========================================
- Coverage    76.02%   76.01%   -0.01%     
===========================================
  Files         1546     1546              
  Lines       165352   165374      +22     
===========================================
+ Hits        125711   125716       +5     
- Misses       39641    39658      +17     

Impacted Files	Coverage Δ
src/goto-programs/string_abstraction.cpp	91.67% <98.50%> (-0.57%)	⬇️
src/util/format_type.cpp	49.23% <0.00%> (-15.39%)	⬇️
src/solvers/smt2/smt2_conv.cpp	66.27% <0.00%> (-0.04%)	⬇️

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