index_exprt enforces array or vector type

[kroening]

The constructors of index_exprt now require that the array operand either
has array or vector type.

• 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/aThe 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 #7018 (d5462fb) into develop (de250f9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #7018   +/-   ##
========================================
  Coverage    77.87%   77.88%           
========================================
  Files         1569     1569           
  Lines       180903   180891   -12     
========================================
- Hits        140882   140878    -4     
+ Misses       40021    40013    -8     

Impacted Files	Coverage Δ
src/ansi-c/expr2c_class.h	100.00% <ø> (ø)
src/goto-instrument/value_set_fi_fp_removal.cpp	100.00% <ø> (ø)
src/goto-programs/restrict_function_pointers.cpp	80.67% <ø> (ø)
jbmc/src/jdiff/jdiff_parse_options.cpp	66.31% <100.00%> (-0.36%)	⬇️
src/ansi-c/expr2c.cpp	67.35% <100.00%> (+<0.01%)	⬆️
.../goto-instrument/goto_instrument_parse_options.cpp	70.86% <100.00%> (-0.07%)	⬇️
src/goto-programs/process_goto_program.cpp	100.00% <100.00%> (ø)
src/goto-programs/remove_function_pointers.cpp	94.33% <100.00%> (+1.54%)	⬆️
src/util/std_expr.h	93.00% <100.00%> (+0.02%)	⬆️
unit/solvers/smt2_incremental/object_tracking.cpp	100.00% <100.00%> (ø)
... and 1 more

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