Fix Optional test for -fb platform

Fix Optional test for -fb platform

Summary:
The Optional test relied on std::string clearing its data when
it is the source of a move. This does not happen for in-situ fbstrings,
so the test breaks in the -fb platform. The solution: wrap the string in
a class that explicitly invalidates its data upon a move.

Test Plan:
fbconfig --platform-all=gcc-4.8.1-glibc-2.17-fb -r folly
fbmake runtests
fbconfig -r folly
fbmake runtests

Reviewed By: tudorb@fb.com

Subscribers: sdwilsh

FB internal diff: D1506840

Tasks: 4943996