Add symbol lookup cache.

Add symbol lookup cache.

gdb/ChangeLog:

	Add symbol lookup cache.
	* NEWS: Document new options and commands.
	* symtab.c (symbol_cache_key): New static global.
	(DEFAULT_SYMBOL_CACHE_SIZE, MAX_SYMBOL_CACHE_SIZE): New macros.
	(SYMBOL_LOOKUP_FAILED): New macro.
	(symbol_cache_slot_state): New enum.
	(block_symbol_cache): New struct.
	(symbol_cache): New struct.
	(new_symbol_cache_size, symbol_cache_size): New static globals.
	(hash_symbol_entry, eq_symbol_entry): New functions.
	(symbol_cache_byte_size, resize_symbol_cache): New functions.
	(make_symbol_cache, free_symbol_cache): New functions.
	(get_symbol_cache, symbol_cache_cleanup): New function.
	(set_symbol_cache_size, set_symbol_cache_size_handler): New functions.
	(symbol_cache_lookup, symbol_cache_clear_slot): New function.
	(symbol_cache_mark_found, symbol_cache_mark_not_found): New functions.
	(symbol_cache_flush, symbol_cache_dump): New functions.
	(maintenance_print_symbol_cache): New function.
	(maintenance_flush_symbol_cache): New function.
	(symbol_cache_stats): New function.
	(maintenance_print_symbol_cache_statistics): New function.
	(symtab_new_objfile_observer): New function.
	(symtab_free_objfile_observer): New function.
	(lookup_static_symbol, lookup_global_symbol): Use symbol cache.
	(_initialize_symtab): Init symbol_cache_key.  New parameter
	maint symbol-cache-size.  New maint commands print symbol-cache,
	print symbol-cache-statistics, flush-symbol-cache.
	Install new_objfile, free_objfile observers.

doc/ChangeLog:

	* gdb.texinfo (Symbols): Document new commands
	"maint print symbol-cache", "maint print symbol-cache-statistics",
	"maint flush-symbol-cache".  Document new option
	"maint set symbol-cache-size".