macros.h already not only defines macros, but also templatized helpers
like {bit_cast} and {arraysize}. Thus {implicit_cast} also belongs
there.

R=tebbi@chromium.org

Bug: v8:7570
Change-Id: Iaea6075dad359d62498453575f22d73ca84e2323
Reviewed-on: https://chromium-review.googlesource.com/1042401
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52991}