[libFuzzer] Allow discarding output in ExecuteCommand in Fuchsia.

Summary:
This commit modifies the way `ExecuteCommand` works in fuchsia by adding
special logic to handle `/dev/null`.

The FuzzerCommand interface does not have a way to "discard" the output,
so other parts of the code just set the output file to `getDevNull()`.
The problem is that fuchsia does not have a named file that is
equivalent to `/dev/null`, so opening that file just fails.

This commit detects whether the specified output file is `getDevNull`,
and if that's the case, it will not copy the file descriptor for stdout
in the spawned process.

NOTE that modifying `FuzzerCommand` to add a "discardOutput" function
involves a significant refactor of all the other platforms, as they all
rely on the `toString()` method of `FuzzerCommand`.

This allows libfuzzer in fuchsia to run with `fork=1`, as the merge
process (`FuzzerMerge.cpp`) invoked `ExecuteCommand` with `/dev/null` as the
output.

Reviewers: aarongreen, phosek

Reviewed By: aarongreen

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D72894

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