[flang][runtime] Support DT edit descriptor in result of READ(SIZE=)

When a formatted I/O READ statement processes a DT edit descriptor to call a
user-defined I/O subroutine to read a derived type data item, all of the
characters that that subroutine reads via child I/O count as charecters
read by an edit descriptor and should accumulate in the result returned
by a SIZE= item in the original READ statement's control list.

Differential Revision: https://reviews.llvm.org/D144234