Runtime: Don't attempt to round-trip mangled names for private types.
Runtime: Don't attempt to round-trip mangled names for private types.

By design, we don't want private or function-nested types to be accessible by mangled name, since they don't have stable identities, and they could inadvertently become ABI if someone serialized a mangled string and expected to deserialize it into a type. Fixes rdar://problem/39826794 .

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