SMT2 back-end: fix inconsistent array flattening

[tautschnig]

Whether we flatten or don't (when datatype support is not available) depends on the expression, but we may need to unflatten when the context expects an array.

Fixes: #8399

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[codecov]

Codecov Report

Attention: Patch coverage is 83.07692% with 11 lines in your changes missing coverage. Please review.

Project coverage is 78.02%. Comparing base (b335979) to head (af46479).
Report is 37 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	82.22%	8 Missing ⚠️
src/solvers/flattening/arrays.cpp	85.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8400   +/-   ##
========================================
  Coverage    78.02%   78.02%           
========================================
  Files         1726     1726           
  Lines       189442   189488   +46     
  Branches     18466    18493   +27     
========================================
+ Hits        147808   147850   +42     
- Misses       41634    41638    +4     

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[TGWDB]

Looks ok, would have been nice to have a test from the original issue.

[rod-chapman]

Can we get this merged please? @kroening @martin-cs