[libFuzzer] Enable tests on Windows

Summary:
Enable tests on Windows and make check-fuzzer pass on it.  Make
check-fuzzer pass on Windows by fixing libFuzzer, fixing tests, and by
disabling tests on Windows. Most of these are disabled temporarily as
support for the tests and the features they test will be added
incrementally.  Other tests will not be enabled since they require
things that are not on Windows (eg: afl_driver tests).  Every test
that was explicitly disabled on Windows has a comment explaining why
(unless obvious like merge-posix.test).

The lit.cfg file was modified to support running tests on windows.
fuzzer-dirs.test was fixed by making the Windows implementation print
the same error message as the posix version.
merge-control-file.test was fixed by making the test binary end with
the ".exe" extension (on all platforms).

Patch By: metzman

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: srhines, mgorny

Differential Revision: https://reviews.llvm.org/D51549

llvm-svn: 341385