Add changelog entry regarding new default C++ standard (C++17). NFC (#17870)

Author: sbc100

ChangeLog.md

@@ -24,6 +24,9 @@ See docs/process.md for more on how version tagging works.
 - Using `-Oz` or `-Os` will no longer pass `-fno-inline-functions` to clang and
   instead rely on clang's normal inline heuristics for these optimization
   levels.  `-fno-inline-functions` can be passed explicitly if needed.
+- C++17 is now the default version of the C++ standard used by the compiler.
+  This is due to an upstream change in llvm.  Use `-std=c++14` (or technically
+  `-std=gnu++14`) to revert to the previous default.
 
 3.1.21 - 09/09/2022
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