writer/msl: Handle texture and sampler variables

Move these module-scope variables to entry point parameters and pass
them as arguments to functions that use them. Disable entry point IO
validation for them.

Emit [[texture()]] and [[sampler()]] attributes on these entry point
parameters.

Fixed: tint:145
Change-Id: I936a80801875a5d0b6cd98a2e8f3e297a2f53509
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53961
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/ast/disable_validation_decoration.cc

@@ -36,6 +36,8 @@ std::string DisableValidationDecoration::InternalName() const {
       return "disable_validation__binding_point_collision";
     case DisabledValidation::kFunctionVarStorageClass:
       return "disable_validation__function_var_storage_class";
+    case DisabledValidation::kEntryPointParameter:
+      return "disable_validation__entry_point_parameter";
   }
   return "<invalid>";
 }

src/ast/disable_validation_decoration.h

@@ -34,6 +34,9 @@ enum class DisabledValidation {
   /// When applied to a function-scoped variable, the validator will not
   /// complain if the storage class is not `function`.
   kFunctionVarStorageClass,
+  /// When applied to an entry-point function parameter, the validator will not
+  /// check for entry IO decorations.
+  kEntryPointParameter,
 };
 
 /// An internal decoration used to tell the validator to ignore specific