Add base/bit_cast.h to follow https://codereview.chromium.org/399313006

(And fix include guard from previous import of base/debug/alias.h.)

Change-Id: I54308d4b3aa2e92263c267d5c309c5adb16eadd0
Reviewed-on: https://chromium-review.googlesource.com/340505
Reviewed-by: Robert Sesek <rsesek@chromium.org>