Warn and immediately throw on naming collisions

[Kukovec]

It may sometimes happen, in a multi-module scenario where module A extends or instantiates module B, that an operator in A has the same name as a bound variable or parameter in B. This causes a slew of shadowing issues (#2452, #2132, #2135).

We should be able to detect this scenario and immediately abort with a warning message, instead of generating hard-to-trace exceptions in one of the passes.