[clang] [clang] add test verifying DR242 behavior (PR #201441)

[Jess Ding via cfe-commits]

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@@ -695,6 +695,37 @@ namespace cwg241 { // cwg241: 9
   }
 } // namespace cwg241
 
+namespace cwg242 { // cwg242: yes
----------------
jessding wrote:

Thanks for your comments! [I tried this](https://godbolt.org/z/MTeTc87hx) on Compiler Explorer and found that clang consistently gives the right error from 2.8 (earliest version I could find) to 3.3. But [the git blame](https://github.com/llvm/llvm-project/blame/83f45b55f8673450d63e9c509507b4d93ada01cd/clang/lib/Sema/SemaNamedCast.cpp#L607) seems to suggest that the relevant code has been in clang since 2008, before the first stable release. Is the convention still to mark the earliest available version here as 2.8? Thank you!

https://github.com/llvm/llvm-project/pull/201441