[flang] Write private symbols to .mod files

We were trying to write only symbols that are part of the public API of
a module to the .mod file. The problem with that is that submodules need
access to the private symbols in their ancestor modules and submodules.

For now, just write out all of the symbols, private or otherwise.
We could be more precise by omitting some private symbols for modules
that don't have separate module procedures and so can't have submodules.

Also, the old implementation went into an infinite loop for recursive
derived types.

This makes CollectSymbols simpler, so have it do both the collecting and
sorting. Make it static so that the types don't have to be defined in
mod-file.h.

Original-commit: flang-compiler/f18@33a10d6238c3b0062c43974114048ccc8cc1f450
Reviewed-on: https://github.com/flang-compiler/f18/pull/177
Tree-same-pre-rewrite: false