iASL: Update error messages for file open errors.

Emit proper filename and abort
For debug file, properly handle stderr after freopen
iASL: Enhancement for constant folding.

Add support to fold expressions with a target operand -- these are
folded and converted to Store:

Add (4, 3, INT1) --> Store (7, INT1)

Also supports ASL+ constructs:

INT1 = 4 + 3 --> Store (7, INT1)