[flang][OpenMP] Parser support for Target directive and Device clause

This patch adds support for the device clause on `Target` directive.
Device clause was added in OpenMP specification version 4.5 to
create a device data environment for the extent of a region. On
target construct, the device expression be either be `ancestor`
(taking after the parent) or assign a new `device_num`.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D126441