[libFuzzer] Add features `windows` and `posix` for lit tests.

Add 2 features: posix and windows.
Sometimes we want some specific tests only for posix and we use:
 REQUIRES: posix
Sometimes we want some specific tests only for windows and we use:
 REQUIRES: windows

Differential Revision: https://reviews.llvm.org/D29418

llvm-svn: 293827