gdb/dwarf: few fixes for handling DW_FORM_{rng,loc}listx

gdb/dwarf: few fixes for handling DW_FORM_{rng,loc}listx

We hit an assertion when loading the binary from PR 26813.  When fixing
it, execution goes a up bit further but then hits another assert, and
another, and another.  With these fours fixes, I am able to load the
binary and get to the prompt.  An error is shown (index pointing outside
of the section), because the DW_FORM_rnglistx attribute is not read
correctly, but that one is taken care of by the next patch.

The four fixes are:

- attribute::form_requires_reprocessing needs to handle forms
  DW_FORM_rnglistx and DW_FORM_loclistx, because set_unsigned_reprocess
  is called for them in read_attribute_value.

- read_attribute_reprocess must call set_unsigned for them, not
  set_address.  The parameter of set_address is a CORE_ADDR, meaning
  it's for program addresses.  Post-reprocess, DW_FORM_rnglistx and
  DW_FORM_loclistx are offsets into their respective sections
  (.debug_rnglists and .debug_loclists).  set_unsigned is the current
  attribute value setter that fits the best.  But perhaps we should have
  a setter that takes a sect_offset?

- read_attribute_process must call as_unsigned_reprocess instead of
  as_unsigned to get the pre-reprocess value, otherwise we hit the
  assert inside as_unsigned that makes sure the attribute doesn't need
  reprocessing.

- attribute::set_unsigned needs to clear the requires_reprocessing flag,
  otherwise it stays set when reprocessing DW_FORM_rnglistx and
  DW_FORM_loclistx attributes.

There's another assert that we hit once the next patch is applied, but
since it's in the same vein as the changes in this patch, I included it
in this patch:

- attribute::form_is_unsigned must handle form DW_FORM_loclistx,
  otherwise we hit the assert when trying to call set_unsigned for an
  attribute of this form.  DW_FORM_rnglistx is already handled.

gdb/ChangeLog:

	PR gdb/26813
	* dwarf2/attribute.h (struct attribute) <set_unsigned>: Clear
	requires_reprocessing flag.
	* dwarf2/attribute.c (attribute::form_is_unsigned): Handle
	DW_FORM_loclistx.
	(attribute::form_requires_reprocessing): Handle DW_FORM_rnglistx
	and DW_FORM_loclistx.
	* dwarf2/read.c (read_attribute_reprocess): Use set_unsigned
	instead of set_address for DW_FORM_loclistx and
	DW_FORM_rnglistx.

Change-Id: I06c156fa3913ca98e4e39085f4ef171645b4bc1e