[clang] [flang] [flang][driver] Introduce FCC_OVERRIDE_OPTIONS. (PR #140556)

[Tarun Prabhu via cfe-commits]

================
@@ -0,0 +1,17 @@
+! RUN: env FCC_OVERRIDE_OPTIONS="#+-Os +-Oz +-O +-O3 +-Oignore +a +b +c xb Xa Omagic ^-###  " %flang -target x86_64-unknown-linux-gnu %s -O2 b -O3 2>&1 | FileCheck %s
----------------
tarunprabhu wrote:

Should this also be `--target`?

```suggestion
! RUN: env FCC_OVERRIDE_OPTIONS="#+-Os +-Oz +-O +-O3 +-Oignore +a +b +c xb Xa Omagic ^-###  " %flang -target x86_64-unknown-linux-gnu %s -O2 b -O3 2>&1 | FileCheck %s
```

https://github.com/llvm/llvm-project/pull/140556