Use new for ada_symbol_cache

Use new for ada_symbol_cache

This changes the ada_symbol_cache to be allocated with 'new' and
managed via unique_ptr.  This simplifies the code somewhat.  Also,
ada_clear_symbol_cache is changed so that it does not allocate a
symbol cache just to clear it.

gdb/ChangeLog
2021-03-02  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (struct ada_symbol_cache) <cache_space>: Now an
	auto_obstack.
	<root>: Initialize.
	(ada_pspace_data): Remove destructor.
	<sym_cache>: Now a unique_ptr.
	(ada_init_symbol_cache, ada_free_symbol_cache): Remove.
	(ada_get_symbol_cache): Use 'new'.
	(ada_clear_symbol_cache): Rewrite.