[builtins] Also simplify the Symbol constructor.

[builtins] Also simplify the Symbol constructor.

No need to rely on the %_IsConstructCall magic here, we can just
implement the Symbol constructor in C++ altogether (it was just a
stupid wrapper around %CreateSymbol anyway).

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1349643002

Cr-Commit-Position: refs/heads/master@{#30762}