Flexible array members: make type checking aware and fix bounds checking

[tautschnig]

Type checking must be ready to handle type changes between declaration
and definition, which b16c7d7 introduced. Also, bounds checking must
not use size_of_expr with these as 24eba85 rightly introduced a
distinction between "sizeof" and the actual object size as mandated by
the C standard.

Fixes: #6787

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• 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.
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[codecov]

Codecov Report

Merging #6788 (22bd66f) into develop (c5f0f36) will increase coverage by 0.00%.
The diff coverage is 96.00%.

@@           Coverage Diff            @@
##           develop    #6788   +/-   ##
========================================
  Coverage    77.00%   77.00%           
========================================
  Files         1594     1594           
  Lines       184350   184371   +21     
========================================
+ Hits        141957   141983   +26     
+ Misses       42393    42388    -5     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.cpp	80.69% <95.23%> (+2.36%)	⬆️
src/ansi-c/goto_check_c.cpp	91.47% <100.00%> (ø)

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