cxa_demangle: fix rvalue ref check

When checking if the type is a r-value ref, we would not do a complete
check.  This would result in us treating a trailing parameter reference
`&)` as a r-value ref, and improperly inject the cv qualifier on the
type.  We now correctly demangle the type `KFvRmE` as a constant
function rather than a constant reference.

Fixes PR31741!

llvm-svn: 292973