writer/msl: Handle private and workgroup variables

Add a transform that pushes these into the entry point and then passes
them by pointer to any functions that need them.

Since WGSL does not allow non-function storage class at
function-scope, add a DisableValidation attribute to bypass this
check.

Fixed: tint/726
Change-Id: Ic1f4cd691a54c19e77a60e8ba178508e4249bfd9
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51962
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/ast/disable_validation_decoration.cc

@@ -34,6 +34,8 @@ std::string DisableValidationDecoration::Name() const {
       return "disable_validation__function_has_no_body";
     case DisabledValidation::kBindingPointCollision:
       return "disable_validation__binding_point_collision";
+    case DisabledValidation::kFunctionVarStorageClass:
+      return "disable_validation__function_var_storage_class";
   }
   return "<invalid>";
 }

src/ast/disable_validation_decoration.h

@@ -31,6 +31,9 @@ enum class DisabledValidation {
   /// When applied to a module-scoped variable, the validator will not complain
   /// if two resource variables have the same binding points.
   kBindingPointCollision,
+  /// When applied to a function-scoped variable, the validator will not
+  /// complain if the storage class is not `function`.
+  kFunctionVarStorageClass,
 };
 
 /// An internal decoration used to tell the validator to ignore specific