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

[Tarun Prabhu via cfe-commits]

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@@ -614,3 +614,28 @@ nvfortran defines `-fast` as
  - `-Mcache_align`: there is no equivalent flag in Flang or Clang.
  - `-Mflushz`: flush-to-zero mode - when `-ffast-math` is specified, Flang will
    link to `crtfastmath.o` to ensure denormal numbers are flushed to zero.
+
+
+## FCC_OVERRIDE_OPTIONS
+
+The environment variable `FCC_OVERRIDE_OPTIONS` can be used to apply a list of
+edits to the input argument lists. The value of this environment variable is
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tarunprabhu wrote:

Can we also document how this works relative to configuration files. For example, will the substitution option `s/XXX/YYY/` be applied to any options added by the configuration files or only to those passed in explicitly by the user?

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