C++ Interop: fix subscript operator tests
C++ structs that define a non-default copy constructor should actually copy their members in the copy constructor:
Swift might copy the struct around, and if it does so after the `setValueAtIndex` call, the modification of `NonTrivialIntArrayByVal::values` won't propagate to the copied struct and these tests no longer pass:
• `Interop/Cxx/operators/member-inline.swift`
• `Interop/Cxx/operators/member-out-of-line.swift`
The tests are currently passing on the main branch despite this, but they might fail after a seemingly unrelated change (which is how I discovered this).
