Add explicit assignment operator definitions to Optional

Add explicit assignment operator definitions to Optional

Summary:
See new test.  Under GCC 4.6 (which is what the folly tests build with) the old code works fine but under 4.7 building fails with:

folly/test/OptionalTest.cpp: In member function ‘virtual void Optional_AssignmentContained_Test::TestBody()’:
folly/test/OptionalTest.cpp:122:14: error: use of deleted function ‘ContainsOptional& ContainsOptional::operator=(const ContainsOptional&)’
folly/test/OptionalTest.cpp:106:21: note: ‘ContainsOptional& ContainsOptional::operator=(const ContainsOptional&)’ is implicitly deleted because the default definition would be ill-formed:
folly/test/OptionalTest.cpp:106:21: error: use of deleted function ‘folly::Optional<int>& folly::Optional<int>::operator=(const folly::Optional<int>&)’
In file included from folly/test/OptionalTest.cpp:17:0:
./folly/Optional.h:84:7: note: ‘folly::Optional<int>& folly::Optional<int>::operator=(const folly::Optional<int>&)’ is implicitly declared as deleted because ‘folly::Optional<int>’ declares a move constructor or move assignment operator
folly/test/OptionalTest.cpp:129:30: error: use of deleted function ‘ContainsOptional& ContainsOptional::operator=(ContainsOptional&&)’
folly/test/OptionalTest.cpp:108:21: note: ‘ContainsOptional& ContainsOptional::operator=(ContainsOptional&&)’ is implicitly deleted because the default definition would be ill-formed:
folly/test/OptionalTest.cpp:108:21: error: non-static data member ‘ContainsOptional::opt_’ does not have a move assignment operator or trivial copy assignment operator
folly/test/OptionalTest.cpp:137:14: error: use of deleted function ‘ContainsOptional& ContainsOptional::operator=(const ContainsOptional&)’

Test Plan:
(1) fbconfig folly/test && fbmake dbg && _build/dbg/folly/test/optional_test
(2) Remove folly/PLATFORM to build with gcc 4.7, then repeat (1).  Without the code fix, the new test fails to build.  With the fix, the test builds and runs fine.

Reviewed By: tjackson@fb.com

FB internal diff: D732402