[flang] More CHARACTER runtime support + unit test

[flang] More CHARACTER runtime support + unit test

Summary:
This is a continuation of development of the runtime support library's functions for
CHARACTER data; specifically, it implements CHARACTER comparisons.
It includes a new unit test, and moves an #include directive to the
runtime's unit testing main header.

Reviewers: tskeith, sscalpone, DavidTruby, jdoerfert

Reviewed By: tskeith

Subscribers: flang-commits, mgorny, llvm-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D78826