[llvm] Document Policy on supporting newer C++ standard in LLVM codebase (PR #156823)

[Angel Cascarino via llvm-commits]

================
@@ -78,6 +78,12 @@ Each toolchain provides a good reference for what it accepts:
 Additionally, there are compiler comparison tables of supported C++ features on
 `cppreference.com <https://en.cppreference.com/w/cpp/compiler_support/17>`_.
 
+To keep track with the evolution of the standard, newer C++ version can be used
----------------
ACascarino wrote:

tiny nitpick:
```suggestion
To keep track with the evolution of the standard, newer C++ versions can be used
```
or 
```suggestion
To keep track with the evolution of the standard, a newer C++ version can be used
```

https://github.com/llvm/llvm-project/pull/156823