Invertibility for unchecked pointers

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#1127

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Fixes #1123

This PR allows pointer arithmetic expressions with unchecked pointer type to be invertible. This means that unchecked pointers with bounds-safe interfaces in unchecked scopes can have inverses.

For example:

void f(int *p : count(1)) _Unchecked {
  p = p + 1;
}

In the body of f, p has the unchecked pointer type int * since it is in an unchecked scope. Before this PR, p would have no inverse in the expression p + 1. After this PR, p has the inverse p - 1.