string-builder.h was using functions defined in *-inl.h and that's not allowed.

BUG=v8:7754,v8:7490

Change-Id: I442ff761f3a5799b60c0d02f7130bf694dca9b1b
Reviewed-on: https://chromium-review.googlesource.com/1145185Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54589}