spirv: Add block decorations with a transform

Any struct which is used as the store type of a buffer variable needs
to have a block decoration. If that struct is nested inside an array
or another struct, we wrap it inside another struct first.

This removes the SPIR-V backend's reliance on the [[block]] attribute,
which will soon be deprecated and removed.

Bug: tint:1324
Change-Id: Ib6ad54f24a3e4a090da9faeed699f266abcb66ba
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/72082
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/resolver/resolver_validation.cc

@@ -2160,7 +2160,8 @@ bool Resolver::ValidateStructure(const sem::Struct* str) {
   }
 
   for (auto* deco : str->Declaration()->decorations) {
-    if (!(deco->Is<ast::StructBlockDecoration>())) {
+    if (!(deco->IsAnyOf<ast::StructBlockDecoration,
+                        ast::InternalDecoration>())) {
       AddError("decoration is not valid for struct declarations", deco->source);
       return false;
     }