[IRGen] Move marker protocol stripping from `mangleTypeSymbol` to `mangleTypeForFlatUniqueTypeRef`
The original check introduced by https://github.com/apple/swift/pull/71855
is too broad. For concrete metadata we call the runtime demangler so
we need to strip off marker protocols when mangling that string and
`mangleTypeForReflection` already does that.
(cherry picked from commit 5d243bd8a2d3fcd7bcc3475bba9a73ff0760b68a)