compiler: Use backend interface for expressions.
* go-gcc.cc (Gcc_backend::nil_pointer_expression): New method.
(Gcc_backend::boolean_constant_expression): New method.
(Gcc_backend::zero_expression): Use this->make_expression rather
than tree_to_expr.
(Gcc_backend::var_expression): Likewise.
(Gcc_backend::integer_constant_expression): Likewise.
(Gcc_backend::float_constant_expression): Likewise.
(Gcc_backend::complex_constant_expression): Likewise.
(Gcc_backend::struct_field_expression): Likewise.
(tree_to_type, tree_to_expr, tree_to_stat): Remove functions.
(tree_to_function, tree_to_block): Remove functions.
(type_to_tree, expr_to_tree, stat_to_tree): Remove functions.
(block_to_tree, var_to_tree, function_to_tree): Remove functions.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@210122 138bc75d-0d04-0410-961f-82ee72b054a4