[clang] [Clang] Implement the core language parts of P2786 - Trivial relocation (PR #127636)

[Nikolas Klauser via cfe-commits]

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@@ -1826,6 +1827,12 @@ The following type trait primitives are supported by Clang. Those traits marked
   functionally equivalent to copying the underlying bytes and then dropping the
   source object on the floor. This is true of trivial types and types which
   were made trivially relocatable via the ``clang::trivial_abi`` attribute.
+* ``__builtin_is_cpp_trivially_relocatable`` (C++): Returns true if an object
+  is trivially relocatable, as defined by the C++26 standard [meta.unary.prop].
+  Note that the caller code should ensure that if the object is polymorphic,
+  the dynamic type is of the most derived type.
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philnik777 wrote:

Why do we introduce another builtin for this instead of fixing the existing one?

https://github.com/llvm/llvm-project/pull/127636