[flang] Add lowering for integer constant

[flang] Add lowering for integer constant

This patch enables the lowering of integer constant.

The `ScalarExprLowering` class is introduced in `ConvertExpr.cpp` to help
the lowering of expression. This patch adds all the placeholder as well for future
expression lowering with the appropriate TODOs.

Integer constant expression are lowered to `arith.constant` with an integer type corresponding to the kind value.

This patch is in support of D118787

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

Reviewed By: schweitz, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D118786