* Improve the logic for determining whether emitting an import lib is
   eligible, and improve the error message when the user provides
   contradictory arguments.
 * Integrate with the EmitLoc / Emit system that already exists, and use
   the `-femit-implib[=path]`/`-fno-emit-implib` convention that already
   exists.
 * Proper integration with the caching system.
 * CLI: fix bug in error reporting for resolving EmitLoc values for
   other parameters.