Introduce new add_psymbol_to_list overload

This adds a new overload of add_psymbol_to_list.  This one takes an
already constructed psymbol and adds it to the bcache and the
appropriate list.

This seemed cleaner than continuing to add parameters to the existing
add_psymbol_to_list, and is more in line with how full symbols are
constructed.

gdb/ChangeLog
2020-04-24  Tom Tromey  <tom@tromey.com>

	* psymtab.c (add_psymbol_to_bcache): Simplify calling convention.
	(add_psymbol_to_list): New overload.  Make old overload call new
	one.
	* psympriv.h (add_psymbol_to_list): New overload.