[flang] External I/O runtime work, repackaged (part 1)

Add a isFixedRecordLength flag member to Connection to
disambiguate the state of "record has known variable length"
from "record has fixed length".  Code that sets and tests this
flag will appear in later patches.  Rearrange data members to
reduce storage requirements, since Connection might indirectly
end up on a program stack frame.  Add a utility member function
BeginRecord(); use it in internal I/O processing.

Reviewed By: tskeith, sscalpone

Differential Revision: https://reviews.llvm.org/D83098