Make snegate / unegate overflow handling consistent with other builtins. (#4744)

Make `int.snegate` ignore the signedness of its operand and
unconditionally check for signed overflow like all the other `int.s*`
builtins do. Fix the prelude implementation of unary `-` for `Core.UInt`
to use `int.unegate` instead of `int.snegate`.

Fix the test for unsigned negate to actually test negating unsigned
integers, and add some tests that unary `-` also works.

toolchain/check/eval.cpp

@@ -783,7 +783,7 @@ static auto PerformBuiltinUnaryIntOp(Context& context, SemIRLoc loc,
 
   switch (builtin_kind) {
     case SemIR::BuiltinFunctionKind::IntSNegate:
-      if (is_signed && op_val.isMinSignedValue()) {
+      if (op_val.isMinSignedValue()) {
         CARBON_DIAGNOSTIC(CompileTimeIntegerNegateOverflow, Error,
                           "integer overflow in negation of {0}", TypedInt);
         context.emitter().Emit(loc, CompileTimeIntegerNegateOverflow,