[flang][hlfir] `hlfir.char_extremum` op definition and codegen

This patch adds an hlfir operation called `char_extremum`, which takes the
lexicographic comparison between a variadic number (minimum of 2 arguments) of
characters.

Discussion for this work can be found in the draft revision found
[here](https://reviews.llvm.org/D143326). The reason I'm not promoting that draft to
a true patch for review was because I needed to separate out the op
definition/codegen and lowering as two separate patches, as preferred by
@jeanPerier.

Differential Revision: https://reviews.llvm.org/D152474