Convert symbol_cache to type-safe registry API

Convert symbol_cache to type-safe registry API

This changes the symbol_cache to use the type-safe registry API.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tom@tromey.com>

	* symtab.c (struct symbol_cache): Add destructor and
	initializers.
	(symbol_cache_key): Move.  Change type.
	(make_symbol_cache, free_symbol_cache): Remove.
	(get_symbol_cache): Update.
	(symbol_cache_cleanup): Remove.
	(ALL_PSPACES, symbol_cache_flush)
	(maintenance_print_symbol_cache)
	(maintenance_print_symbol_cache_statistics, _initialize_symtab):
	Update.