[flang] Include analyzed exprs in parse tree dump

When dumping the parse tree, if there are analyzed expressions,
assignments, or call statements, include the Fortran form of those
in the dump.

This uses the same `AnalyzedObjectsAsFortran` object that unparsing
does so it applies to the same nodes.

We already do something like this for `Name` nodes as well as some
predefined types, so abstract it into an `AsFortran` function which
returns the Fortran source for a node that is to be included in the
dump.

Original-commit: flang-compiler/f18@823b044d2a009f43fe5ce15755a8ae23f70dfed8
Reviewed-on: https://github.com/flang-compiler/f18/pull/878
Tree-same-pre-rewrite: false