[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`).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374684 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed