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

[Fangrui Song via cfe-commits]

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+! 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
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MaskRay wrote:

`--target=x86_64-unknown-linux-gnu` instead of the long deprecated `-target `

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