[flang][runtime] Return the right mutable modes from INQUIRE in child I/O

During child I/O -- a call to a user-defined subroutine with a generic interface
to handle read and write operations on a specific derived type -- ensure that
any INQUIRE statement probing mutable modes like BLANK= will observe changes
that have been made to those modes via control edit descriptors like BN
while processing the original I/O statement's format.

Differential Revision: https://reviews.llvm.org/D144046