[flang] Add runtime interface for GET_COMMAND_ARGUMENT

GET_COMMAND_ARGUMENT takes a lot of optional arguments: VALUE, LENGTH,
STATUS and ERRMSG. This patch breaks up the interface into 2 different
functions:

* One for getting the LENGTH of an argument.

* One for getting the VALUE and the ERRMSG of an argument. This returns
the STATUS, which can be easily ignored by lowering if it is missing in
the invocation.

Differential Revision: https://reviews.llvm.org/D108688