Add a more general version of lookup_struct_elt_type.

Add a more general version of lookup_struct_elt_type.

lookup_struct_elt is a new function which returns a tuple of
information about a component of a structure or union.  The returned
tuple contains a pointer to the struct field object for the component
as well as a bit offset of that field within the structure.  If the
field names a field in an anonymous substructure, the offset is the
"global" offset relative to the original structure type.  If noerr is
set, then the returned tuple will set the field pointer to NULL to
indicate a missing component rather than throwing an error.

lookup_struct_elt_type is now reimplemented in terms of this new
function.  It simply returns the type of the returned field.

gdb/ChangeLog:

	* gdbtypes.c (lookup_struct_elt): New function.
	(lookup_struct_elt_type): Reimplement via lookup_struct_elt.
	* gdbtypes.h (struct struct_elt): New type.
	(lookup_struct_elt): New prototype.