[Hexagon] Add defaulted operator= to classes with defaulted copy ctor

This avoids deprecation warning:
```
warning: definition of implicit copy assignment operator for 'AddrInfo'
is deprecated because it has a user-declared copy constructor
[-Wdeprecated-copy]
```

This fixes https://github.com/llvm/llvm-project/issues/57229

(cherry picked from commit 252cea03)