[Mangler] Simplify the mangling into a stream. NFC.
[Mangler] Simplify the mangling into a stream. NFC.

This is a tiny cleanup that makes it easier to add the compression/decompression layers.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift