[flang] DATA statement processing (part 2/4): Initial images

Summary:
Defines a representation for the initialized memory image of
a variable.  This image is populated by DATA statement
processing as designator elements are put into correspondence
with values, then converted into an initializer in the symbol
table so that lowering can pass the initial image to the
code generator.

Reviewers: tskeith, PeteSteinfeld, sscalpone, jdoerfert, DavidTruby

Reviewed By: tskeith

Subscribers: mgorny, llvm-commits, flang-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D82131