folly: OptionalTest.cpp: suppress a -Wself-move warning

folly: OptionalTest.cpp: suppress a -Wself-move warning

Summary:
Clang's new -Wself-move warning dings this code:

b = std::move(b);

But in this case, that is a deliberate test of
this capability, so we suppress that particular
warning for the duration of this small test function.

Test Plan:
Run this and note that it now compiles: (it passes the tests, too)
fbconfig --clang --with-project-version=clang:dev -r folly/test:optional_test && fbmake dbgo

Reviewed By: watashi@fb.com

Subscribers: folly-diffs@, yfeldblum

FB internal diff: D1849689

Tasks: 6244745

Signature: t1:1849689:1423877932:4f479d682bd8ba97dcabed7fc29ce0a22c013969

@build-break (using this to work around bogus push failure: https://phabricator.fb.com/P19730606)