Support DW_TAG_rvalue_reference type

Support DW_TAG_rvalue_reference type

Make gdb DWARF reader understand the DW_TAG_rvalue_reference type tag. Handling
of this tag is done in the existing read_tag_reference_type() function, to
which we add a new parameter representing the kind of reference type
(lvalue vs rvalue).

gdb/ChangeLog

	PR gdb/14441
	* dwarf2read.c (process_die, read_type_die_1): Handle the
	DW_TAG_rvalue_reference_type DIE.
	(read_tag_reference_type): Add new parameter "refcode".