[clang] [clang-tools-extra] [libcxx] [clang] fix diagnostic printing of expressions ignoring LangOpts (PR #134693)

[Erich Keane via cfe-commits]

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@@ -7379,6 +7379,14 @@ class RecoveryExpr final : public Expr,
   friend class ASTStmtWriter;
 };
 
+/// Insertion operator for diagnostics.  This allows sending
+/// Expr into a diagnostic with <<.
+inline const StreamingDiagnostic &operator<<(const StreamingDiagnostic &DB,
+                                             const Expr *E) {
+  DB.AddTaggedVal(reinterpret_cast<uint64_t>(E), DiagnosticsEngine::ak_expr);
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erichkeane wrote:

As far as correctness: we need to smuggle pointers through a `uintptr_t`.  Beyond that, there is no harm then again casting that immediately to the `uint64_t`.

So the 'fixit' suggestion is:
```suggestion
  DB.AddTaggedVal(static_cast<uint64_t>(reinterpret_cast<uintptr_t>(E)), DiagnosticsEngine::ak_expr);
```

The underlying type here is fine, and SHOULD stay `uint64_t`.  But the `reinterpret_cast` of pointer-to-int itself is the problematic part.  If you'd like, you can put a static-assert that `sizeof(uintptr_t)` is <= `sizeof(uint64_t)`, but I'm not horribly worried about 128 bit pointers

https://github.com/llvm/llvm-project/pull/134693