[CIR] Upstream pointer subtraction handling

[andykaylor]

Overview

Upstream support for subtracting pointer values using the cir.ptr_diff operation. This will also require upstreaming the code to lower cir.ptr_diff to LLVM IR.

Suggested minimal test case

unsigned long long test_ptr_diff(int *a, int* b) {
  return a - b;
}

Existing incubator tests

clang/test/CIR/CodeGen/ptrdiff.c
clang/test/CIR/CodeGen/ptrdiff.cpp

[andykaylor]

@kimsh02 Would you like to work on this?

[kimsh02]

@andykaylor Yes, thanks!