[Serialization] Don't warn when a deserialized category is equivalent to an existing one.

A single class allows multiple categories to be defined for it. But if
two of such categories have the same name, we emit a warning. It's not a
hard error but a good indication of a potential mistake.

With modules, we can end up with the same category in different modules.
Diagnosing such a situation has little value as the categories in
different modules are equivalent and don't reflect the usage of the same
name for different purposes. When we deserialize a duplicate category,
compare it to an existing one and warn only when the new one is
different.

rdar://104582081

Differential Revision: https://reviews.llvm.org/D144149