[flang] Lower basic STOP statement

This patch lowers STOP statement without arguments
and ERROR STOP. STOP statement with arguments lowering will
come in later patches ince it requires some expression lowering
to be added.
STOP statement is lowered to a runtime call.

Also makes sure we are creating a constant in the MLIR arith constant.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan, schweitz

Differential Revision: https://reviews.llvm.org/D118697



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>