Equip bv_pointerst::postponedt with a constructor

[tautschnig]

This permits constructing list entries in place rather than first
creating a dummy element and then replacing each member.

• Each commit message has a non-empty body, explaining why the change was made.
• 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 #6140 (df6e264) into develop (763f6a2) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6140      +/-   ##
===========================================
+ Coverage    74.90%   74.95%   +0.05%     
===========================================
  Files         1447     1447              
  Lines       158198   158209      +11     
===========================================
+ Hits        118491   118593     +102     
+ Misses       39707    39616      -91     

Impacted Files	Coverage Δ
src/goto-instrument/code_contracts.h	93.75% <ø> (ø)
src/goto-instrument/code_contracts.cpp	85.45% <100.00%> (+0.10%)	⬆️
src/solvers/flattening/bv_pointers.cpp	81.03% <100.00%> (-0.24%)	⬇️
src/solvers/flattening/bv_pointers.h	100.00% <100.00%> (ø)
src/util/c_types.h	100.00% <100.00%> (+1.80%)	⬆️
src/ansi-c/gcc_types.cpp	41.86% <0.00%> (-46.52%)	⬇️
src/ansi-c/scanner.l	40.71% <0.00%> (-21.02%)	⬇️
src/ansi-c/literals/parse_float.cpp	83.65% <0.00%> (-12.50%)	⬇️
src/util/find_symbols.cpp	82.22% <0.00%> (-3.34%)	⬇️
src/ansi-c/c_nondet_symbol_factory.cpp	97.70% <0.00%> (-2.30%)	⬇️
... and 28 more

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