[clang] [clang] Implement a bitwise_copyable builtin type trait. (PR #86512)

[Ilya Biryukov via cfe-commits]

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@@ -1120,6 +1120,14 @@ class QualType {
   /// Return true if this is a trivially copyable type (C++0x [basic.types]p9)
   bool isTriviallyCopyableType(const ASTContext &Context) const;
 
+  /// Return true if the type is safe to bitwise copy by memcpy.
+  ///
+  /// This is an extension in clang: bitwise clonable types act as trivially
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ilya-biryukov wrote:

We should update the documentation to talk about the things that we are actually checking now, i.e. that `memcpy` will succeed and have a valid object representation, even if the lifetime is not formally started. 
Mentioning when it's not the case would also be helpful:
- memcpy fails, e.g. sanitizers that add padding with poison.
- (potentially) types with ObjC lifetimes.

https://github.com/llvm/llvm-project/pull/86512