gdb/dwarf2: update call_site::target comment

This comment refers to the field location kind enum, even though call
sites were moved to their own enum in 7eb21cc ("Change
call_site_target to use custom type and enum").  Update it.

Change-Id: I089923170c919853efb2946529221a4b55e720c1

Author: simark

gdb/dwarf2/call-site.h

@@ -198,7 +198,7 @@ struct call_site
   struct call_site *tail_call_next = nullptr;
 
   /* * Describe DW_AT_call_target.  Missing attribute uses
-     FIELD_LOC_KIND_DWARF_BLOCK with FIELD_DWARF_BLOCK == NULL.  */
+     m_loc_kind == DWARF_BLOCK with m_loc.dwarf_block == nullptr.  */
 
   struct call_site_target target {};