[libc++] Quick fix to remove a narrowing conversion from the in_out_result test.

This needs a proper solution in a follow-up. The issue is that the
Standard defines conversions between `in_out_result` classes with
different template types as just `return {in, out};`. Because the
expression uses list initialization, it will fail to compile if the
conversion happens to be narrowing -- which is probably unintended.

Surprisingly, this error wasn't caught by the CI.

Differential Revision: https://reviews.llvm.org/D117089