[NFC] Separate out GlobalTypeRewriter::mapTypeNames (#6829)

Previously a module's type names were updated in
`GlobalTypeRewriter::rebuildTypes`, which builds new versions of the
existing types, rather than `GlobalTypeRewriter::mapTypes`, which
otherwise handles replacing old types with new types everywhere in a
module, but should not necessarily replace names. So that users of
`mapTypes` who are building their own versions of existing types can
also easily update type names, split type name mapping logic out into a
new method `GlobalTypeRewriter::mapTypeNames`.

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