[BoundsWidening] Implement the bounds widening dataflow analysis

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#1093

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This is an updated implementation of the dataflow analysis for bounds widening.
In addition to the existing support for bounds widening on a conditional
dereferencing a null-terminated array at its upper bound this implementation
also supports bounds widening in presence of _Where clauses. For example:

  _Nt_array_ptr<char> p : bounds(p, p + 1);
  int x = strlen(p) _Where p : bounds(p, p + x); // Bounds of p widened to bounds(p, p + x).

[secure-sw-dev-bot]

Comment from @mgrang:

Superseded by checkedc/checkedc-clang#1122