Handling C style casts in checking free variables

[dopelsunce]

void f() {
  int a1 checked[5];
  short int t20 : byte_count(5 * sizeof(int)) = (short int)a1;
}

a1 and t20 are incorrectly reported as free variables by #903.

This causes one test to fail: checkedc/tests/typechecking/bounds.c

When checking the assignment,
EquivExprs contains the set { LValueToRValue(t20), (short int)LValueToRValue(a1) } and we're searching for the variable LValueToRValue(a1). Thus a1 and t20 are treated unequal.

Fix: this could be part of #909 to add DeclRefExprs to the variable set, and then search the EquivExprs for an expression that is the DeclRefExpr ignoring casts and parentheses.

Basically add a IsRValueCastOfVariableIgnoringCasts method that would return true for (short int)LValueToRValue(a1) and DeclRefExpr(a1)

[kkjeer]

Fixed by 867cf58.