[Bug default/25449] Factor out compilation units

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=25449

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #0)
> So, as a first step we could do a optimization in DWZ to look at all the
> items that are selected to be moved into a PU, and decide whether we can
> transform the PU into a CU and drop the imports.

One of the requirements probably has to be that the items are from a single
language.

> An #include directive appearing outside any other declarations is a good
> candidate to be represented using DW_TAG_compile_unit.
> 
> However, an #include appearing inside a C++ namespace declaration or a
> function, for example, is not a good candidate because the entities included
> are not necessarily file level entities.

The appendix suggests DIEs in a namespace are not good candidates, but I think
what that tries to say is that if we originally we have a DIE in a namespace:
...
DIE2: compilation unit B
  DIE3: namespace bla
    DIE1
...
and do some factoring out like so:
...
DIE0: factored-out unit A
  DIE1
DIE2: compilation unit B
  DIE3: namespace bla
    DIE4: import DIE0
...
the factored-out unit cannot use DW_TAG_compile_unit, because DIE1 is not a
globally visible entry.

However, dwz generates this type of partial unit:
...
DIE0: partial unit A
  DIE3: namespace bla
    DIE1
DIE2: compilation unit B
  DIE4: import DIE0
...
which basically works around this problem, and I don't see a reason here why
unit A can't be a compilation unit.

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