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

[Sam McCall via cfe-commits]

================
@@ -526,6 +526,7 @@ TYPE_TRAIT_2(__is_layout_compatible, IsLayoutCompatible, KEYCXX)
 #include "clang/Basic/TransformTypeTraits.def"
 
 // Clang-only C++ Type Traits
+TYPE_TRAIT_1(__is_bitwise_copyable, IsBitwiseCopyable, KEYCXX)
----------------
sam-mccall wrote:

this new trait should be documented in `docs/LanguageExtensions`.

The documentation needs to explicitly mention whether or not the user code needs to perform a "start lifetime" operation to incarnate the resulting bytes. 

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