Expose llvm groupdata size as a shadergroup attribute

src/include/OSL/oslexec.h

@@ -443,6 +443,7 @@ class OSLEXECPUBLIC ShadingSystem {
     ///   string entry_layers[]      List of entry point layers.
     ///   string pickle              Retrieves a serialized representation
     ///                                 of the shader group declaration.
+    ///   int llvm_groupdata_size    Size of the GroupData struct.
     /// Note: the attributes referred to as "string" are actually on the app
     /// side as ustring or const char* (they have the same data layout), NOT
     /// std::string!

src/liboslexec/shadingsys.cpp

@@ -2125,6 +2125,10 @@ ShadingSystemImpl::getattribute(ShaderGroup* group, string_view name,
         *(void**)val = n ? &group->m_userdata_init_vals[0] : NULL;
         return true;
     }
+    if (name == "llvm_groupdata_size" && type == TypeDesc::TypeInt) {
+        *(int*)val = (int)group->llvm_groupdata_size();
+        return true;
+    }
 
     return false;
 }