[flang] Update preprocessing document, commentary

[flang] Update preprocessing document, commentary

Extend documentation on preprocessing

IsSimplyContiguous

basic skeleton

Apply suggested improvements to mod-file.cc

Checks for new call13.f90 (15.4.2.2 explicit interfaces)

Implement checking for procedures that can be called via an implicit interface

Argument checking, test fixing

Better argument checking

Better derived type compatibility checking

Treat externals as if implicitly interfaced

Extend IEEE_EXCEPTIONS module so tests still pass with argument checking

SAME_TYPE_AS and EXTENDS_TYPE_OF intrinsic inquiry functions

Define interfaces for most intrinsic subroutines

Better PASS arguments

More with PASS() argument

Prep for review

address comments

fix comment

Remove formatted expression from specification expression error message per review

more review comments

Original-commit: flang-compiler/f18@3cca775da9302e3a2acec4a34800cd7666899b5f
Reviewed-on: https://github.com/flang-compiler/f18/pull/776