transform: Fix PromoteInitializersToConstVar handling of for-loops

PromoteInitializersToConstVar was erroring on for loops that contained array or structure constructor expressions.

Added lots more tests.

Fixed: tint:1364
Change-Id: I033eaad94756ea496fc8bc5f03f39c6dba4e3a88
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/75580
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

test/statements/for/initializer/basic.wgsl.expected.hlsl

@@ -0,0 +1,11 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  {
+    [loop] for(int i = 0; ; ) {
+    }
+  }
+}