cplus_demangle_fill_component: Handle DEMANGLE_COMPONENT_RVALUE_REFERENCE

This patch almost a decade ago:

...
    2007-08-31  Douglas Gregor  <doug.gregor@gmail.com>

        * cp-demangle.c (d_dump): Handle
        DEMANGLE_COMPONENT_RVALUE_REFERENCE.
        (d_make_comp): Ditto.
...

... missed doing the same change to cplus_demangle_fill_component that
was done to d_make_comp.  I.e., teach it to only validate that we're
not passing in a "right" subtree.  GDB has recently (finally) learned
about rvalue references, and a change to make it use
cplus_demangle_fill_component more ran into an assertion because of
this.

(GDB is the only user of cplus_demangle_fill_component in both the gcc
and binutils-gdb trees.)

libiberty/ChangeLog:
2017-03-27  Pedro Alves  <palves@redhat.com>

	* cp-demint.c (cplus_demangle_fill_component): Handle
	DEMANGLE_COMPONENT_RVALUE_REFERENCE.

From-SVN: r246502