Frontend: Don't accept null DiagnosticsEngines when building ASTUnits

The various ways to create an ASTUnit all take a refcounted pointer to
a diagnostics engine as an argument, and if it isn't pointing at
anything they initialize it. This is a pretty confusing API, and it
really makes more sense for the caller to initialize the thing since
they control the lifetime anyway.

This fixes the one caller that didn't bother initializing the pointer
and asserts that the argument is initialized.