[llvm] [llvm][docs] Correct the list of the available -fveclib= options to match with the reality (PR #168205)

[Paul Osmialowski via llvm-commits]

pawosm-arm wrote:

> We have StringRef::equals_insensitive

We do. But using `==` operator for a string comparison always looks better. I won't be arguing on this though, as I'm seeing it used frequently throughout the frontend driver code. But then, what would you do about this?

```
  // List of veclibs which when used with -fveclib imply -fno-math-errno.
  constexpr std::array VecLibImpliesNoMathErrno{llvm::StringLiteral("ArmPL"),
                                                llvm::StringLiteral("SLEEF")};

...

    case options::OPT_fveclib:
      VecLibArg = A;
      NoMathErrnoWasImpliedByVecLib =
          llvm::is_contained(VecLibImpliesNoMathErrno, A->getValue());
      if (NoMathErrnoWasImpliedByVecLib)
        MathErrno = false;
```

There's no `is_contained_case_insensitive` available, so what would be a good LLVM idiom for handling this?

NB: this is one of the places I spotted, I wonder how many other places will be affected non-obvious way if we allow case insensitive spelling of the names which were used in their current forms for the last few years...


https://github.com/llvm/llvm-project/pull/168205