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

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NagyDonat wrote:

Since suggesting that "matrix" testcase I realized that it won't work, because `ArrayBoundV2` doesn't perform bounds checking for the lower-dimensional sub-arrays of a multidimensional array (it only checks that the accessed memory location is within the full array). For example in `int matrix[10][10];` it reports that `matrix[1][200]` overflows the bounds of "matrix", but doesn't report `matrix[1][50]`. I tried to eliminate this limitation, but that patch had to be reverted because the meaning of `ElementRegion` is ambiguous.

Due to this limitation, it's a bit difficult to ensure that `ArrayBoundV2` calls `getDescriptiveName()` on an `ElementRegion`, but I think adding a `FieldRegion` layer should work:
```
struct {
  int numbers[10];
} table[100];
int func(int x, int y) {
  int tmp = x;
  x = y;
  y = tmp;
  return table[x].numbers[-1]; 
}
```
Here I expect that the checker would (incorrectly) report something like `Access of 'table[y].numbers' at negative byte offset`.


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