[llvm] Add const check (PR #181190)

[Dmitrii Makarenko via llvm-commits]

Devjiu wrote:

@d0k  looks like 
```
/vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/llvm/unittests/ADT/ArrayRefTest.cpp:492:5: error: static assertion failed due to requirement 'std::is_constructible_v<llvm::MutableArrayRef<int>, std::span<int, 18446744073709551615>>': should be able to construct MutableArrayRef from mutable std::span
  492 |     std::is_constructible_v<MutableArrayRef<int>, std::span<int>>,
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
705.300 [802/17/503] Building CXX object unittests/ADT/CMakeFiles/ADTTests.dir/APFixedPointTest.cpp.o
```

I've broken this check with span. I don't know how MutableArrayRef should work with rvalues - as it doesn't own data, so moving to the constructor with `constexpr MutableArrayRef(C &&V) `, and binding to temporaries would risk dangling references.
We can add specific constructor for span or modify the static check somehow. 

https://github.com/llvm/llvm-project/pull/181190