Allow multiple partial symbol readers per objfile

Allow multiple partial symbol readers per objfile

This patch finally changes gdb so that an objfile can have multiple
sources of partial symbols (or mixed partial symbols and other kinds
of indices).

This is done by having each symbol reader create its own
psymbol_functions object and add it to the 'qf' list in the objfile.

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

	* xcoffread.c (xcoff_initial_scan): Create partial symtabs.
	* symfile.c (syms_from_objfile_1, reread_symbols): Update.
	* psymtab.h (make_psymbol_functions): Don't declare.
	* psymtab.c (make_psymbol_functions): Remove.
	(maintenance_print_psymbols): Update.
	* psympriv.h (struct psymbol_functions): Add no-argument
	constructor.
	* objfiles.h (struct objfile) <reset_psymtabs>: Remove.
	<partial_symtabs>: Remove.
	* mdebugread.c (mdebug_build_psymtabs): Create partial symtabs.
	* elfread.c (read_partial_symbols): Update.
	(elf_symfile_read): Remove check for existing partial symbols.
	Don't clear "qf".
	* dwarf2/read.c (dwarf2_has_info): Remove check for existing
	partial symbols.
	(dwarf2_build_psymtabs): Add psymbol_functions parameter.  Create
	partial symtabs.
	* dwarf2/public.h (dwarf2_build_psymtabs): Add psymbol_functions
	parameter.
	* dbxread.c (dbx_symfile_read): Create partial symtabs.
	* ctfread.c (elfctf_build_psymtabs): Create partial symtabs.