[libcxx] Fix memory leak in strstream tests.

Summary: The strstream function `str()` sets `freeze(true)`. When `freeze` is true the destructor is not allowed to free any dynamically allocated memory. The memory leak causes ASAN to fail on these tests. To ensure memory is deallocated `strstream.freeze(false)` is called at the end of the tests.

Reviewers: danalbert, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6261