demangler: Handle <=> operator in the demangler [PR94797]

The demangler didn't handle spaceship operator.

2020-04-27  Jakub Jelinek  <jakub@redhat.com>

	PR demangler/94797
	* cp-demangle.c (cplus_demangle_operators): Add ss <=> operator.
	* testsuite/demangle-expected: Add operator<=> test.