[flang] Upstream partial lowering of COMMAND_ARGUMENT_COUNT intrinsic

This patch adds partial lowering of the "COMMAND_ARGUMENT_COUNT" intrinsic
to the backend runtime hook implemented in patch D109048. Also adds a
"helper" function for retrieving the default integer type from
FIRBuilder, which will be used later when finishing the lowering of
intrinsic.

Differential Revision: https://reviews.llvm.org/D117869