Change clear_program_space_solib_cache to method on program_space

Change clear_program_space_solib_cache to method on program_space

This changes clear_program_space_solib_cache to be a method on
program_space.  Also, it removes a call to this function from the
program_space destructor, as that is not necessary.

gdb/ChangeLog
2020-10-29  Tom Tromey  <tom@tromey.com>

	* progspace.c (program_space::~program_space): Don't call
	clear_program_space_solib_cache.
	(program_space::clear_solib_cache): Rename from
	clear_solib_cache.
	* solib.c (handle_solib_event): Update.
	* progspace.h (struct program_space) <clear_solib_cache>: New
	method.
	(clear_program_space_solib_cache): Don't declare.