There isn't much that uses bit_cast. bit_cast requires memcpy which
requires <string.h>

On Mac, <string.h> includes AvailabilityInternal.h which is half a
megabyte and defines dozens of macros. Providing <string.h> in
base/macros.h also tempts IWYU violations. Try to avoid it.

Add base/bit_cast.h and include it from the 17 files that need it. Then
remove the <string.h> #include from base/macros.h and IWYU.

BUG=395001

Review URL: https://codereview.chromium.org/399313006

Cr-Commit-Position: refs/heads/master@{#369348}