[Diagnostics] Implement -Wsizeof-pointer-div

Summary:
void test(int *arr) {
    int arr_len = sizeof(arr) / sizeof(*arr);  // warn, incorrect way to compute number of array elements
}

Enabled under -Wall (same behaviour as GCC)

Reviewers: rsmith, MTC, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: MTC, thakis, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D52949

llvm-svn: 345847