Make the TurboFan Typer infer proper types for calls to the known
Symbol.prototype.toString() and Symbol.prototype.valueOf() builtins.

Bug: v8:8015
Change-Id: Ia133bd8f2cfcabaf984820a953b357d1747d6892
Reviewed-on: https://chromium-review.googlesource.com/1213203
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55719}