writer/hlsl: Inline fallthrough case statements

FXC does not support fallthrough case statements (DXC does).

Fixed: tint:1082
Change-Id: I82e1add5455e438056259f773f34bf9db05970b4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60480
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>

test/switch/fallthrough.wgsl.expected.hlsl

@@ -1,11 +1,12 @@
-SKIP: FAILED
-
 [numthreads(1, 1, 1)]
 void f() {
   int i = 0;
   switch(i) {
     case 0: {
       /* fallthrough */
+      {
+        break;
+      }
     }
     default: {
       break;
@@ -13,5 +14,3 @@ void f() {
   }
   return;
 }
-C:\src\tint\test\Shader@0x000001AF1A4F6940(5,5): error X3533: non-empty case statements must have break or return
-