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

[Fangrui Song via cfe-commits]

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@@ -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
----------------
MaskRay wrote:

I noticed that clang/docs doesn't have the documentation. Perhaps we should document `clang/docs/UsersManual.rst` as well and reference it from FlangDriver.md?

(`"Each edit should be one of the following forms:" list ` should probably reference the clang doc)

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