[flang][runtime] Advance output record in specific case (#167786)
When a formatted WRITE takes place in a defined output subroutine called
from a context in which record advancement is allowed, such as NAMELIST,
the char-string-edit-descs in the format can trigger record advancement.
Also clean up confusing messiness lingering from the separation of
iostat.h two headers in flang/.../Runtime. iostat.h didn't need to be
put into flang/.../Runtime since it's included only by flang-rt, and
iostat-consts.h doesn't need one of its includes.
Fixes https://github.com/llvm/llvm-project/issues/167757.