Annotate flexible array members

[tautschnig]

Make sure we can distinguish flexible array members from other arrays,
for example when wanting to distinguish their compile-time from their
runtime size.

This is in preparation of #6661.

• 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 #6663 (24eba85) into develop (1154f13) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6663      +/-   ##
===========================================
+ Coverage    76.92%   77.09%   +0.16%     
===========================================
  Files         1583     1582       -1     
  Lines       183314   182650     -664     
===========================================
- Hits        141018   140817     -201     
+ Misses       42296    41833     -463     

Impacted Files	Coverage Δ
jbmc/src/janalyzer/janalyzer_parse_options.cpp	48.75% <100.00%> (ø)
jbmc/src/jbmc/jbmc_parse_options.cpp	76.02% <100.00%> (ø)
jbmc/src/jdiff/jdiff_parse_options.cpp	67.74% <100.00%> (-0.35%)	⬇️
src/analyses/goto_check.cpp	81.39% <100.00%> (+17.75%)	⬆️
src/analyses/goto_check_c.cpp	91.60% <100.00%> (-0.01%)	⬇️
src/ansi-c/c_typecheck_type.cpp	77.08% <100.00%> (ø)
.../goto-instrument/goto_instrument_parse_options.cpp	69.39% <100.00%> (+0.03%)	⬆️
src/goto-programs/process_goto_program.cpp	100.00% <100.00%> (ø)
src/util/pointer_offset_size.cpp	92.56% <100.00%> (+0.04%)	⬆️
src/util/simplify_expr.cpp	85.56% <100.00%> (+0.01%)	⬆️
... and 2 more

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[martin-cs]

Seems reasonable. I guess testing will be via another PR, probably #6661 ?