Change add_psymbol_to_list to use an enum

This changes add_psymbol_to_list to use an enum, rather than a pointer
to a vector, to decide where to put the new symbol.  This reduces the
number of direct references to the static_psymbols and global_psymbols
members of the objfile, which is handy in a later patch.

gdb/ChangeLog
2019-01-10  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (scan_xcoff_symtab): Update.
	* psymtab.c (add_psymbol_to_list): Replace "list" parameter with
	"where".
	* mdebugread.c (parse_partial_symbols)
	(handle_psymbol_enumerators): Update.
	* dwarf2read.c (add_partial_symbol, load_partial_dies): Update.
	* dbxread.c (read_dbx_symtab): Update.
	* psympriv.h (psymbol_placement): New enum.
	(add_psymbol_to_list): Update.