Allow conversion between T* and Cpp.void*. (#6575)

Support an implicit conversion from `T*` to `Cpp.void*` and to `const
Cpp.void*`, and an `unsafe as` conversion in the opposite direction.

In order to support C++ calls taking and returning `void*` (which get
mapped to Carbon `Optional(Cpp.void*)`, also support conversions from
`Optional(T)` to `Optional(U)` if there's a conversion from `T` to `U`.

Fix a bug in `OptionalStorage` for `T*` where its `HasValue` was exactly
backwards.

toolchain/lower/handle_call.cpp

@@ -505,6 +505,11 @@ static auto HandleBuiltinCall(FunctionContext& context, SemIR::InstId inst_id,
                                     context.GetValue(arg_ids[0])));
       return;
     }
+
+    case SemIR::BuiltinFunctionKind::PointerUnsafeConvert: {
+      context.SetLocal(inst_id, context.GetValue(arg_ids[0]));
+      return;
+    }
   }
 
   CARBON_FATAL("Unsupported builtin call.");