Using slim RW lock on windows
Using slim RW lock on windows

Done as performance improvement.
(in our text parser case we see ~2% improvement vs std:mutex)

Change-Id: Ifc01aa1590849618f46624cec4f71031688ca730
Reviewed-on: https://code-review.googlesource.com/c/re2/+/52850
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2