Check if format argument is identifier to avoid error err-emit
Fixes#139104
When `argument` is not an identifier, it should not be considered a field access. I checked this and if not emit an invalid format string error. I think we could do with a little finer error handling, I'll open an issue to track this down later.
The first commit submits the ui test, the second commits the code and the changes to the test output.
r? compiler
9c949b0373