Add dwarf2_per_objfile parameter to cutu_reader's constructors
The cutu_reader type is used for reading the CU represented by the
passed dwarf2_per_cu_data object. This reading is done in the context
of a given obfile, which is currently the one associated to the passed
dwarf2_per_cu_data object. Since the dwarf2_per_cu_data type will
become objfile-independent, we will need to pass the objfile separately.
This patch therefore adds a dwarf2_per_objfile parameter to the
cutu_reader constructors, as well as to their callers, up until the
point where we can get the dwarf2_per_objfile object from somewhere
else. In the end, this allows removing the reference to
dwarf2_per_cu_data::dwarf2_per_objfile in cutu_reader::cutu_reader.
A few dwarf2_per_cu_data::dwarf2_per_objfile references are added (e.g.
in dwarf2_fetch_die_type_sect_off). This is temporary, this will be
removed once these functions will get re-worked in subsequent patches.