Reference psymtabs via per_bfd in DWARF reader

Reference psymtabs via per_bfd in DWARF reader

This changes the DWARF reader to reference the psymtabs via the
per_bfd object, rather than via the objfile.  This helps isolate the
partial symtabs from the objfile.

gdb/ChangeLog
2021-03-20  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (dwarf2_create_include_psymtab): Add per_bfd
	parameter.
	(process_psymtab_comp_unit_reader)
	(build_type_psymtab_dependencies, dwarf2_build_psymtabs_hard)
	(add_partial_subprogram, dwarf2_ranges_read, dwarf_decode_lines):
	Reference psymtabs via per_bfd.