[clang] [KeyInstr][Clang][NFC] Don't set -dwarf-use-key-instructions (PR #144115)

[Orlando Cazalet-Hyams via cfe-commits]

================
@@ -8,8 +8,14 @@
 // HELP-NOT: key-instructions
 
----------------
OCHyams wrote:

I'm not sure that's right - if I change `key-instructions` to `clang` I get:
```
/home/och/dev/llvm-project/clang/test/DebugInfo/KeyInstructions/flag.cpp:8:14: error: HELP-NOT: excluded string found in input
// HELP-NOT: clang
             ^
<stdin>:1:11: note: found here
OVERVIEW: clang LLVM compiler
```

(Either way - I've noticed that my RUN lines were not running, which I've now fixed)

https://github.com/llvm/llvm-project/pull/144115