String refinement: support pointers

[tautschnig]

We need to use bv_pointerst to convert address_of expressions.

Please review commit-by-commit: this PR includes cleanup to reduce the number of uses of null_message_handlert, which can hide errors. A particular example of such errors was the use of boolbvt instead of bv_pointerst in string refinement.

• 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 #6915 (b861c23) into develop (6685497) will increase coverage by 0.00%.
The diff coverage is 89.53%.

@@           Coverage Diff            @@
##           develop    #6915   +/-   ##
========================================
  Coverage    77.78%   77.79%           
========================================
  Files         1568     1568           
  Lines       180269   180290   +21     
========================================
+ Hits        140230   140252   +22     
+ Misses       40039    40038    -1     

Impacted Files	Coverage Δ
...straint_instantiation/instantiate_not_contains.cpp	0.00% <0.00%> (ø)
src/goto-instrument/wmm/goto2graph.cpp	54.57% <ø> (+0.07%)	⬆️
...rs/strings/string_concatenation_builtin_function.h	0.00% <ø> (ø)
src/solvers/strings/string_constraint_generator.h	100.00% <ø> (ø)
src/solvers/strings/string_dependencies.h	100.00% <ø> (ø)
...c/solvers/strings/string_format_builtin_function.h	66.66% <ø> (ø)
...olvers/strings/string_insertion_builtin_function.h	81.81% <ø> (ø)
src/xmllang/graphml.cpp	66.20% <ø> (+0.45%)	⬆️
src/xmllang/graphml.h	100.00% <ø> (ø)
...strings/string_constraint_generator_comparison.cpp	76.87% <75.00%> (-0.05%)	⬇️
... and 13 more

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