Add support for `Buffer.from(new String('...'))` and
`Buffer.from({[Symbol.toPrimitive]() { return '...'; }})`
PR-URL: https://github.com/nodejs/node/pull/13725
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
