[flang][runtime] Corrections to formatted child I/O

A handful of I/O statements (OPEN, CLOSE, positioning) are not allowed
on units during child I/O; catch violations and report errors.
Also finesse error handling during FORMAT runtime parsing of DT
derived type edit descriptors, and ensure that formatted child
I/O is nonadvancing.

Differential Revision: https://reviews.llvm.org/D145751