[libFuzzer] Avoid undefined behavior, properly discard output to stdout/stderr.

Fix libFuzzer when setting -close_fd_mask to a non-zero value.
In previous implementation, libFuzzer closes the file descriptors for
stdout/stderr. This has some disavantages:

For `fuzzer-fdmask.test`, we write directly to stdout and stderr using the
file streams stdout and stderr, after the file descriptors are closed, which is
undefined behavior. In Windows, in particular, this was making the test fail.

Also, if we close stdout and we open a new file in libFuzzer, we get the file
descriptor 1, which could generate problem if some code assumes file descriptors
refers to stdout and works directly writing to the file descriptor 1, but it
will be writing to the opened file (for example using std::cout).

Instead of closing the file descriptors, I redirect the output to /dev/null on
linux and nul on Windows.

Differential Revision: https://reviews.llvm.org/D28718

llvm-svn: 292743