Add dialect option subscript-check to relax bound checks for ODO items (FR #437)

[nberth]

Only ODO items for now, which actually makes this PR quite short. I'm not sure what is to be relaxed about reference modifications. If that stays like this, -fcheck-record-bound may not be the clearest of names; maybe -frelax-odo-bound-checks would suit better.

https://sourceforge.net/p/gnucobol/feature-requests/437/

[codecov-commenter]

Codecov Report

Merging #99 (ea9aff0) into gcos4gnucobol-3.x (7ba3fb5) will increase coverage by 0.00%. The diff coverage is 100.00%.

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@@                Coverage Diff                 @@
##           gcos4gnucobol-3.x      #99   +/-   ##
==================================================
  Coverage              65.11%   65.12%           
==================================================
  Files                     31       31           
  Lines                  58307    58313    +6     
  Branches               15349    15350    +1     
==================================================
+ Hits                   37969    37975    +6     
  Misses                 14412    14412           
  Partials                5926     5926           

Impacted Files	Coverage Δ
cobc/flag.def	100.00% <100.00%> (ø)
cobc/typeck.c	64.99% <100.00%> (+0.02%)	:arrow_up:

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[nberth]

Ok thanks for all the hints and specs. Now upstream