[Bug symtab/28200] DW_AT_ranges handling in partial_die_info::read does not handle discontinuous ranges

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

https://sourceware.org/bugzilla/show_bug.cgi?id=28200

--- Comment #2 from Tom de Vries <vries at gcc dot gnu.org> ---
Essentially, there are two interpretations of has_pc_info:
- this is not something to be ignored, build a symbol in the partial symbol
  table
- the field lowpc and highpc are valid.

The code:
...
            if (dwarf2_ranges_read (ranges_offset, &lowpc, &highpc, cu,
                                    nullptr, tag))
              has_pc_info = 1;
...
uses the first interpretation to fix the problem that a symbol is not listed in
the partial symbol table.

Which also implies the second interpretation, which apparently usually doesn't
cause much trouble, but evidently it does in combination with this workaround
for gcc 4.8 (see PR15231):
...
          scan_partial_symbols (first_die, &lowpc, &highpc,
                                cu_bounds_kind <= PC_BOUNDS_INVALID, cu);
...

So, at this point I see two potential solutions:
- add an address map along side lowpc/highpc and pass that instead of nullptr
to 
  dwarf2_ranges_read, and handle things from there-on
- split up the two interpretations of has_pc_info, say by adding
  highlowpc_valid (or have_ranges), and when encountering DW_AT_ranges
  just mark highlowpc_valid = 0 and has_pc_info = 1, and deferring reading
  the ranges till we have the partial symtab address map to work with.

My guess is that the latter is more space-efficient.

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