The original fix in 21574bdfa9
was incomplete, as it only addressed the TInstance types, but
not their linked (typ.constructor) TConstructor instances.
This would (potentially among other issues) cause assertion
errors in llvm_ir_generator due to the wrong associated globals
being referenced; see added test case for an example that
previously caused such a crash.
Also modified the name collision detection from O(len(type_map))
(so quadratic overall in the number of custom types) to cache
names in sets for O(1) lookup.
81d2b37b57