Add prototypes for layout_record and layout_union.

Add prototypes for layout_record and layout_union.

(smallest_mode_for_size): New function.
(variable_size): Allow variable sizes if global_bindings_p returns a negative
value.
(mode_for_size): Fix incorrect comment.
(layout_decl): If DECL has a size already, don't copy from type.
(layout_type, case INTEGER_TYPE, ENUMERAL_TYPE): Use smallest integer mode
that fits, whether or not it fits exactly.

From-SVN: r4776