This requires forcibly toggling the 'willHaveBody' value of the Decl that's
being built up before/after handling the attributes in order to query
this property on the Decl.
This also updates a WASM test that seemed incorrect (the attribute is
only allowed on function declarations, not definitions).
This seems like a hack to work around what feels like a bug -- if the declarator knows the function is a definition, then why does the FunctionDecl AST node claim the function won't have a body? It seems strange to me that we don't set that bit when acting on the function declarator but instead wait until we're acting on the start of the function definition to set it; does anything break if you start setting that flag earlier?