disallow switch case capture discards

Previously Zig allowed you to write something like,
```zig
switch (x) {
    .y => |_| {
```

This seems a bit strange because in other cases, such as when
capturing the tag in a switch case,
```zig
switch (x) {
    .y => |_, _| {
```
this produces an error.

The only usecase I can think of for the previous behaviour is
if you wanted to assert that all union payloads are able
to coerce,
```zig
const X = union(enum) { y: u8, z: f32 };

switch (x) {
    .y, .z => |_| {
```

This will compile-error with the `|_|` and pass without it.

I don't believe this usecase is strong enough to keep the current
behaviour; it was never used in the Zig codebase and I cannot
find a single usage of this behaviour in the real world, searching
through Sourcegraph.

test/behavior/switch_loop.zig

@@ -270,3 +270,30 @@ test "switch loop on non-exhaustive enum" {
     try S.doTheTest();
     try comptime S.doTheTest();
 }
+
+test "switch loop with discarded tag capture" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest;
+
+    const S = struct {
+        const U = union(enum) {
+            a: u32,
+            b: u32,
+            c: u32,
+        };
+
+        fn doTheTest() void {
+            const a: U = .{ .a = 10 };
+            blk: switch (a) {
+                inline .b => |_, tag| {
+                    _ = tag;
+                    continue :blk .{ .c = 20 };
+                },
+                else => {},
+            }
+        }
+    };
+    S.doTheTest();
+    comptime S.doTheTest();
+}

test/cases/compile_errors/switch_loop_discarded_capture.zig

@@ -0,0 +1,16 @@
+export fn foo() void {
+    const S = struct {
+        fn doTheTest() void {
+            blk: switch (@as(u8, 'a')) {
+                '1' => |_| continue :blk '1',
+                else => {},
+            }
+        }
+    };
+    S.doTheTest();
+    comptime S.doTheTest();
+}
+
+// error
+//
+// :5:25: error: discard of capture; omit it instead