[clang] [llvm] Adapted MemRegion::getDescriptiveName to handle ElementRegions (PR #85104)

[via cfe-commits]

NagyDonat wrote:

In addition to the above-mentioned issues there is also a conceptual problem with using `getOriginRegion()` to describe a symbol: it names the region where the symbol _originated_ (if it originated as the unknown initial value of a symbol), which is not necessarily the region where the symbol is stored _now_.

For example, if a function `void func(int x, int y)` swaps the values stored in 

For example consider the (somewhat contrived) code 
```
int matrix[10][10];
int func(int x, int y) {
  int tmp = x;
  x = y;
  y = tmp;
  return matrix[x][-999]; 
}
```
Here the checker alpha.security.ArrayBoundV2 (which uses `getDescriptiveName`) would report that
```
Access of 'matrix[y]' at negative byte offset
```
because the symbolic value of the index _originated from_ the parameter `y` (i.e. the symbol is identified as "this is the initial value of `y`).

https://github.com/llvm/llvm-project/pull/85104