[flang] Clause 13 (Input/output editing) syntax and semantic checking (flang-compiler/f18#498)

* Clause 13 (Input/output editing) syntax and semantic checking

Diagnose:
 - all clause syntax errors (independent of normal parsing)
 - all clause constraints
 - use of non-standard extensions under -Mstandard
 - obvious program requirement violations

The code:
 - is invoked for both format statement and I/O statement constant formats
 - should be useable by the runtime I/O library for runtime formats
 - is able to recover from errors
 - is able to diagnose multiple errors in a single format
 - has accurate markers for errors
 - can process (fixed character size) Hollerith strings
 - generates reasonable error messages for known error scenarios
 - should not require C++ run-time library support

The code is templatized for use with fixed size character kind={1,2,4} input,
but only the kind=1 variant is actually exercised.

* Review update.

* Review update.

* Review update.

* Improve error processing of unterminated formats.

Original-commit: flang-compiler/f18@c04b7518df75bb516f039065d4edcba87dae8210
Reviewed-on: https://github.com/flang-compiler/f18/pull/498