Summary:
It weakens canonical path in order to avoid an explosion of locations when a struct type has pointers to struct.
For example:
```
struct Tree {
struct Tree *root;
struct Tree *left;
struct Tree *right;
};
```
It was able to generate lots of abstract locations before this diff:
```
t->root
t->left
t->right
t->root->left
t->root->right
t->left->root
t->left->right
t->right->root
t->right->left
t->root->left->right
t->root->right->left
t->left->root->right
t->left->right->root
t->right->root->left
t->right->left->root
```
By this diff, pointer fields that have the same type are (unsoundly) canonicalized to the same one. For example,
```
t->root
t->root->left
t->root->right
t->root->left->right
t->root->right->left
```
are canonicalized to `t->root`. This is definitely unsound but I believe it is better than the location explosion by which analyses do not terminate in a reasonable time or giving a fixed limit of depth to the field access.
Reviewed By: mbouaziz
Differential Revision: D13503808
fbshipit-source-id: 867018712
Sungkeun Cho