Bounds Widening: Fix handling of bounds variables for nt_array_ptrs

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#898

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There were 3 issues with the handling of variables used in the declared bounds
of an nt_array_ptr:

1. Bounds variables were being determined only for nt_array_ptrs declared in
   the current block. As a result, if a variable is killed in a block then the
   widened bounds in the successor blocks could not be killed. This is fixed by
   storing the bounds variables along with the nt_array_ptrs for the entire
   function instead of per block.

2. Bounds variables for function parameters were not getting determined
   correctly. This is fixed by uniforming the logic to determine bounds
   variables for function parameters and local variables.

3. Bounds declared as CountBoundsExpr were not being handled. This is fixed by
   reading the NormalizedBounds instead of invoking ExpandBoundsToRange.

Also performed minor NFC code cleanups.

Fixes checkedc/checkedc-clang#895