Many Clang options are treated as unknown arguments in release_18x branch

[shivaramaarao]

These options gives error when clang built through release18.x branch is used
-mavx
-mavx2
-mavx512
-mfma
-funroll-loops
-fno-strict-aliasing
-mtune=core-avx2

There may be some more, but if the fix is general one, it might solve issues with other options as well.

[bryanpkc]

A possible generalized fix is to modify TableGen to treat all Clang options as Flang options, but I thought this was too permissive, and may lead to the driver listing a lot of incorrect or unsupported options. The current approach is to add the FlangOption visibility to specific options that we know are supported and needed by Classic Flang.

I believe that the current situation with the LLVM Flang driver (flang-new) is the same:

https://flang.llvm.org/docs/FlangDriver.html#adding-new-compiler-options

[bryanpkc]

A fix has been merged. Please re-open if other Clang options are needed in the Classic Flang driver.