d: Fix segfault in build_frontend_type on alpha-*-*

d: Fix segfault in build_frontend_type on alpha-*-*

The va_list type for Alpha includes a nameless dummy field for alignment
purposes.  To transpose this into D, a field named "__pad%d" is inserted
into the struct definition.

It was also noticed that in the D front-end AST copy of the backend
type, all offsets for fields generated by build_frontend_type were set
to zero due to a wrong assumption that DECL_FIELD_OFFSET would have a
non-zero value.  This has been fixed to use byte_position instead.

gcc/d/ChangeLog:

	* d-builtins.cc (build_frontend_type): Handle struct fields with NULL
	DECL_NAME.  Use byte_position to get the real field offset.