[clang] [C] Add diagnostic + attr for unterminated strings (PR #137829)

[Aaron Ballman via cfe-commits]

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@@ -5024,3 +5024,9 @@ def OpenACCRoutineDecl :InheritableAttr {
     void printPrettyPragma(raw_ostream &OS, const PrintingPolicy &Policy) const;
   }];
 }
+
+def NonString : InheritableAttr {
+  let Spellings = [GCC<"nonstring">];
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AaronBallman wrote:

We don't typically do that. `GCC` spelling gets `__attribute__((foo))` and `[[gnu::foo]]` while the `Clang` spelling gets you `__attribute__((foo))` and `[[clang::foo]]`, so we usually go with `Clang` when Clang is the first to implement the attribute, and `GCC` when GCC is the first to implement, and only do both spellings in the very unlikely and terrible situation where the semantics are different between the two spellings. We do have `ClangGCC` spelling used by the sanitizers and that suggests we might want to change our policy, but that's orthogonal to this PR.

https://github.com/llvm/llvm-project/pull/137829