[ASTGen] Start using 'TokenSyntax.rawText'
`TokenSyntax.text` causes intiation of heap allocated `Swift.String` for
each access. `TokenSyntax.rawText` is faster than that because it's just
a reference to the pre-allocated buffer. Also, converting to
`BridgedString` is simplar, and guaranteed to be free, unlike bridging
via `String.withUTF8(_:)`
