[libc++][test] Silence MSVC warning in std::optional test

`make_optional<string>(4, 'X')` passes `4` (an `int`) as the first argument to `string`'s `(size_t, charT)` constructor, triggering a signed/unsigned mismatch warning when compiling with MSVC at `/W4`. The incredibly simple fix is to instead use an unsigned literal (`4u`).

llvm-svn: 374684