When we moved the String.fromCharCode builtin to C++, we slightly
regressed the fast single character code argument case. Recovered some
of the performance by implementing the builtin using the TurboFan
CodeStubAssembler.

Drive-by-fix: Make sure the stack trace from the implicit ToNumber
conversion in String.fromCharCode includes the builtin by adding a
regression test for that.

R=yangguo@chromium.org
BUG=chromium:609831,chromium:613947,v8:5049

Review-Url: https://codereview.chromium.org/2021143003
Cr-Commit-Position: refs/heads/master@{#36611}