Use unique_ptr for htabs

Use unique_ptr for htabs

This patch updates the type-conversion caching in C compile to use
unique pointers.  This patch also removes the on-demand allocation of the
symbol error map in favor of initialization, simplifying the code.

gdb/ChangeLog
        * compile/compile-internal.h (compile_instance::~compile_instance):
        Remove calls to htab_delete.
        <m_type_map, m_symbol_err_map>: Switch type to htab_up.
        * compile.c (compile_instance::compile_instance): Initialize
        htab unique pointers.
        (compile_instance::get_cached_type, compile_instance::insert_type)
        (compile_instance::error_symbol_once): Update for unique_ptr.