[clang] [clang] Add flag for relaxed pointer subtraction (PR #196392)

[via cfe-commits]

eleviant wrote:

> The test added here is causing a build failure on a PR that doesn't touch related code. See: https://github.com/llvm/llvm-project/actions/runs/29825976924/job/88619657659?pr=208747#step:9:19635
> 
> ```
> # | S:\clang\test\CodeGen\ptr-subtract-stable.c:4:17: error: CHECK-LABEL: expected string not found in input
> # | // CHECK-LABEL: i64 @sub(ptr noundef %p, ptr noundef %q)
> # |                 ^
> # | <stdin>:1:1: note: scanning from here
> # | ; ModuleID = 'S:\clang\test\CodeGen\ptr-subtract-stable.c'
> # | ^
> # | <stdin>:7:79: note: possible intended match here
> # | define dso_local noundef range(i64 -2305843009213693952, 2305843009213693952) i64 @sub(ptr noundef %0, ptr noundef %1) local_unnamed_addr #0 {
> ```
> 
> So it looks like at least the argument names were lost.

Have just fixed it

https://github.com/llvm/llvm-project/pull/196392