Changes to location reporting in dwarflint

[Petr Machata <pmachata at redhat dot com>]

Hi there,

a rather large patch has landed on dwarf branch.  This is a summary for
those interested in writing dwarflint checks that are not me (hi Mark!)

The goal was to clean up the way we create location descriptors for
reporting (the "wheres").  Until now we used single "struct where" with
a bunch of fields: section, some addresses, and chain links.  Because it
turned out that we need to pinpoint various odd messages (such as .rel*
sections, location expressions), section_id ended up holding a bunch of
values that were not debuginfo-related at all.  Then there was the added
wrinkle of CU DIE being formatted differently from normal DIEs in case
of aranges.  It was all a bit hacky and full of exceptions.

So this is now gone.  Instead, we have a top-level abstract "class
locus" with a single interface "std::string format" and several
sub-classes that take care of details.  They are all used as structures,
the abstract class is only necessary for passing generic loci references
around, and ultimately to wr_message or wr_error.  But if there ever is
a need to hold a bunch of distinct loci in one array, they new'ing and
later deleting them as is usual in C++ is obviously possible.

Most formatting stayed the way it was, but DIE and abbrev loci in
particular were changed to report the attribute name as soon as it is
known.  This should make citations more clear:

  warning: .debug_abbrev: abbr. 0x11, attr. endianity: attribute from later DWARF version.
  error: .debug_info: DIE 0x29, attr. decl_file: references .debug_line table, but CU DIE lacks DW_AT_stmt_list.

The interesting locus subclasses probably are section_locus, for forming
arbitrary section pointers, and die_locus, for forming DIE references.
die_locus can be initialized simply by debug_info_entry:

      for (dwarf::debug_info_entry::children_type::const_iterator
	   jt = children.begin (); jt != children.end (); ++jt)
	// ...
	wr_message (die_locus (*jt), mc_impact_3 | mc_die_other) // ...

Other loci subclasses (e.g. arange_locus) can be extended to allow
similar initialization from high-level dwarf structures.  Since they are
just a bunch of classes implementing an interface, with comparatively
little magic involved, it should be pretty straightforward.

PM