The copy and move assignment operators weren't returning anything and
were hence producing "non-void function does not return a value"
warnings.
As the test doesn't actually need a definition for these operators, I've
removed the definition and simply declared them.
Currently, trying to do this causes an assertion failure in SILGen, so the
corresponding line in the SILGen test is commented out.
See https://bugs.swift.org/browse/SR-12750
This imports const members of C++ structs/classes stored properties with
an inaccessible setter.
Note that in C++ there are ways to change the values of const members,
so we don't use `WriteImplKind::Immutable` storage.
Resolves: [SR-12463](https://bugs.swift.org/browse/SR-12463)