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

[via cfe-commits]

R-Goc 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. 

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