Adjust GDB to demangler API change

Before commit 3a8724032abf, DEMANGLE_COMPONENT_CAST was used for both
casts and conversion operators.  We now have
DEMANGLE_COMPONENT_CONVERSION for the latter.

gdb/ChangeLog:
2014-11-28  Pedro Alves  <palves@redhat.com>

	* cp-name-parser.y (conversion_op): Use
	DEMANGLE_COMPONENT_CONVERSION instead of DEMANGLE_COMPONENT_CAST.