[clang] [clang][driver] Suppress gnu-line-marker when saving temps (PR #134621)

[Fangrui Song via cfe-commits]

================
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -std=c99 -fsyntax-only -pedantic %s 2>&1 | grep 'warning: this style of line directive is a GNU extension'
----------------
MaskRay wrote:

We prefer `FileCheck` to `grep`. FileCheck enables testing the line number, e.g.

```
// CHECK: [[#@LINE+1]]:4: warning: `
# 0 "zero"
```

https://github.com/llvm/llvm-project/pull/134621