Sema: add error for switch on slice

Closes #12651

src/Sema.zig

@@ -9003,6 +9003,9 @@ fn zirSwitchCond(
         .ErrorSet,
         .Enum,
         => {
+            if (operand_ty.isSlice()) {
+                return sema.fail(block, src, "switch on type '{}'", .{operand_ty.fmt(sema.mod)});
+            }
             if ((try sema.typeHasOnePossibleValue(block, operand_src, operand_ty))) |opv| {
                 return sema.addConstant(operand_ty, opv);
             }

test/cases/compile_errors/switch_on_slice.zig

@@ -0,0 +1,13 @@
+pub export fn entry() void {
+    var a: [:0]const u8 = "foo";
+    switch (a) {
+        "--version", "version" => unreachable,
+        else => {},
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:13: error: switch on type '[:0]const u8'