[flang] Support lowering of intrinsic GET_COMMAND

As Fortran 2018 16.9.82, all the arguments of GET_COMMAND are optional.
When they are all absent, do nothing so to be consistent with gfortran
and ifort. The semantic analysis and runtime have been supported.

This intrinsic was introduced from F2003, and this supports the lowering
of it.

Reviewed By: PeteSteinfeld, jeanPerier

Differential Revision: https://reviews.llvm.org/D137887