Add explicit (void) cast to unused unique_ptr::release() results

Summary:
This patch adds explicit `(void)` casts to discarded `release()` calls to suppress -Wunused-result.

This patch fixes *all* warnings are generated as a result of [applying `[[nodiscard]]`  within libc++](https://reviews.llvm.org/D26596).
Similar fixes were applied to Clang in r286796.

Reviewers: chandlerc, dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26598

llvm-svn: 286797