[dmd-internals] [DWARF] Correct approach for adding module/import support

[Iain Buclaw via dmd-internals]

Hi, I've received no comment whatsoever despite repeated pings.

https://github.com/D-Programming-Language/dmd/pull/4746
https://github.com/D-Programming-Language/dmd/pull/4750
https://github.com/D-Programming-Language/dmd/pull/4773

This is pretty much the rough summary of what I want to mimic in DMD.

PR #4746:
- Modules are emitted as DW_TAG_module
- External modules (not being compiled) are correctly marked as external.
- Module being emitted are marked as having children, and all symbols
that are part of that module are emitted into it, with exception to
- Symbols that are not extern(D), in which case they are declared as
being outside the module.

PR #4750:
- Imported modules, selective imports, and renamed imports are emitted
as DW_TAG_imported_declaration, this requires that
- All imported modules and declarations for the current compiled
module are declared in debug, as extern symbols unless we are
compiling for them too.

PR #4773:
- Associate a DW_TAG_module to a Symbol, which is in turn associated
with a Module
- Re-introduce ModuleInfoDeclaration, and move the generation of
ModuleInfo out of Module and into the correctly named frontend type.


PR 4746 is trivial as all modules are emitted as one-at-a-time.  There
are only two non-trivial-but-simple-enough cases to address in the PR:
- Firstly, extern(C) or extern(C++) symbols, which must be delayed
until after the module has finished.
- Secondly, all extern(D) statics and __gshared symbols nested in
functions/types, which must be emitted as a child of the module, not a
child of the enclosing function/type.

PR 4750 is more difficult as it requires that we now must keep track
of Modules.  So we now need a way to associate them with a backend
Symbol.

PR 4773 is an attempt at implementing the first part of what is needed
for #4750, but it needs some direction.

Any input?  I'm at the stage where I will be submitting patches to
gdb, if nothing is done on the compiler side, then debugging
experience of DMD compiled programs will go downhill very quickly.

Iain.