[flang] Progress on Fortran I/O runtime

[flang] Progress on Fortran I/O runtime

Use internal units for internal I/O state

Replace use of virtual functions

reference_wrapper

Internal formatted output to array descriptor

Delete dead code

Begin list-directed internal output

Refactorings and renamings for clarity

List-directed external I/O (character)

COMPLEX list-directed output

Control list items

First cut at unformatted I/O

More OPEN statement work; rename class to ExternalFileUnit

Complete OPEN (exc. for POSITION=), add CLOSE()

OPEN(POSITION=)

Flush buffers on crash and for terminal output; clean up

Documentation

Fix backquote in documentation

Fix typo in comment

Begin implementation of input

Refactor binary floating-point properties to a new header, simplify numeric output editing

Dodge spurious GCC 7.2 build warning

Address review comments

Original-commit: flang-compiler/f18@9c4bba11cf2329575ea9ee446f69e9caa797135c
Reviewed-on: https://github.com/flang-compiler/f18/pull/982