[flang] Make subscript list argument a nullable pointer

[flang] Make subscript list argument a nullable pointer

Component::CreatePointerDescriptor unconditionally expects a
vector of subscripts to be passed as an argument, but is called
from NAMELIST input with a null pointer.  Make that argument
a nullable pointer, move it to the end of the argument list,
and give it a default value of nullptr.

Differential Revision: https://reviews.llvm.org/D113312