[clang] [analyzer] Generalize MemRegion::getDescriptiveName (PR #213991)

[Benedek Kaibas via cfe-commits]

benedekaibas wrote:

@NagyDonat This is a great change, thank you so much! When I implemented the `getDescriptiveName()` for the lifetime checkers (currently in alpha stage) I faced the same issue:
> in cases when there was no exact name and Memregion::getDescriptiveName just returned an empty string.

I came up with the same `getRegionName` and I got the inspiration for that from the `ArrayBoundChecker`. 

Now the lifetime checkers can eliminate the wrapper as well which makes the code so much cleaner. Thanks for this change, it is so great!

https://github.com/llvm/llvm-project/pull/213991