[flang] DATA stmt processing (part 1/4): designator folding

Summary:
Add code to resolve constant Designators at compilation time
into a base Symbol, byte offset, and field size.  This is used in
later DATA statement processing to identify the static storage being
initialized by each object in a DATA statement.  Also implement
the reverse mapping so that Designators can be reconstructed for
use in error messages about (e.g.) duplicate initializers.

Reviewers: tskeith, PeteSteinfeld, sscalpone, jdoerfert, DavidTruby

Reviewed By: PeteSteinfeld

Subscribers: mgorny, llvm-commits, flang-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D82125