[flang][runtime] Allow record advancement in child I/O via '/' control edit descriptor

[flang][runtime] Allow record advancement in child I/O via '/' control edit descriptor

My earlier misreading of the Fortran standards had convinced me that child I/O
-- meaning the use of user-defined subroutines via generic interfaces to implement
data transfer statements -- was not allowed to advance the current record in the
ultimate unit of the original (non-child parent) data transfer statement.
This turns out to be wrong, so forward AdvanceRecord() from ChildFormattedIoStatement<>
to its parent I/O statement rather than implementing it as a no-op.

Differential Revision: https://reviews.llvm.org/D144205