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

[Tarun Prabhu via cfe-commits]

================
@@ -614,3 +614,30 @@ 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
+a space separated list of edits to perform. These edits are applied in order to
+the input argument lists. Edits should be one of the following forms:
----------------
tarunprabhu wrote:

What do you think of this?
```suggestion
The environment variable `FCC_OVERRIDE_OPTIONS` can be used to edit flang's
command line arguments. The value of this variable is a space-separated list of
edits to perform. The edits are applied in the order in which they appear in `FCC_OVERRIDE_OPTIONS`. Each edit should be one of the following forms:
```

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