Update comments in scan_partial_symbols and add_partial_subprogram

Update comments in scan_partial_symbols and add_partial_subprogram

I read comment of scan_partial_symbols about NEED_PC and how *LOWPC
and *HIGHPC are updated:

   DW_AT_ranges).  If NEED_PC is set, then this function will set
   *LOWPC and *HIGHPC to the lowest and highest PC values found in CU
   and record the covered ranges in the addrmap.

NEED_PC is only used in the callee of scan_partial_symbols,
add_partial_subprogram,

  if (pdi->tag == DW_TAG_subprogram)
    {
      if (pdi->has_pc_info)
        {
          if (pdi->lowpc < *lowpc)
            *lowpc = pdi->lowpc;
          if (pdi->highpc > *highpc)
            *highpc = pdi->highpc;
	  if (need_pc)

*LOWPC and *HIGHPC is updated regardless of NEED_PC.  When NEED_PC is
true, addrmap is updated.  It would be clear to rename NEED_PC to
SET_ADDRMAP.  That is what this patch does.  Beside this, this patch
also adjust comments in related functions.

gdb:

2014-08-24  Yao Qi  <yao@codesourcery.com>

	* dwarf2read.c (scan_partial_symbols):  Update comments.
	Rename argument 'need_pc' with 'set_addrmap'.
	（add_partial_namespace): Rename argument 'need_pc' with
	'set_addrmap'.
	(add_partial_module): Likewise.
	(add_partial_subprogram): Likewise.  Update comments.
	(dwarf2_name): Fix typo.