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.
Sorry for the commented includes. I'll update the patch to remove them