change in begin_borrow instruction in latest SIL version

[Abdulraheem-Arar]

In the newer SIL version these 2 attributes have been added which are causing the SIL parser to fail when they are not handled.

The optional pointer_escape attribute specifies that a pointer to the operand escapes within the borrow scope introduced by this begin_borrow.

The optional var_decl attribute specifies that the operand corresponds to a local variable in the Swift source.

These two attributes should be handled also by the SIL parser so the case class for the begin_borrow instruction should be altered.

these attributes are not currently in the SIL parser as this is the case class for the begin_borrow at the moment.
case class beginBorrow(lexical: Boolean, operand: SILOperand) extends SILOperator