[clang] Explicitly document that `__is_trivially_relocatable(T)` implies that `T` is implicit-lifetime.

[devin.jeanpierre]

Summary

Previous change: D114732

The wording here might need some tweaking. What I want to guarantee is that everything valid for an implicit-lifetime type is also valid for these types. Either they're added to the set of implicit-lifetime types as an extension, or the operations are valid for additional non-implicit-lifetime types -- something like that.

Something to this effect is required for it to be defined behavior to memcpy to a new location, and then use the type at that new location. And so, this guarantee is necessary for __is_trivially_relocatable to be actually useful in practice. See e.g. D119385 for how this plays out.

AIUI, Clang is already fine with this and needs no code changes. Indeed, we see many types which are not what the standard would call implicit-lifetime, yet which use memcpy for copying: std::vector only checks for trivial move, and does not pair this with requiring trivial destroy. (See e.g. __construct_forward_with_exception_guarantees)

(Specifically: I believe that Clang is already fine with treating any non-polymorphic type as if it were implicit-lifetime, but I am not sure how to check this belief. There's not much in the way of explicit references to "implicit-lifetime" types in the codebase. Had a brief out of band conversation with rsmith, but may of course have misunderstood the conclusions here.)

[• Quuxplusone]

FWIW, I think this is a good idea. I even think that the parentheses and "Note:" are too self-deprecating, and this deserves to be a bigger deal. Perhaps __is_implicit_lifetime(T) should be an intrinsic in its own right! Then you could say very concretely that __is_trivially_relocatable(T) implies __is_implicit_lifetime(T).

[devin.jeanpierre]

Rebase off of D119385. Sorry, I'm bad at git.

clang/docs/LanguageExtensions.rst

 * ``__is_trivial`` (C++, GNU, Microsoft, Embarcadero)
 * ``__is_trivially_assignable`` (C++, GNU, Microsoft)
 * ``__is_trivially_constructible`` (C++, GNU, Microsoft)
 * ``__is_trivially_copyable`` (C++, GNU, Microsoft)
 * ``__is_trivially_destructible`` (C++, MSVC 2013)
 * ``__is_trivially_relocatable`` (Clang): Returns true if moving an object
   of the given type, and then destroying the source object, is known to be
   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.
+  source object on the floor. (Note: any such type is, therefore, implicit-lifetime.)
+  This is true of trivial types and types which were made trivially relocatable
+  via the ``clang::trivial_abi`` attribute.
 * ``__is_union`` (C++, GNU, Microsoft, Embarcadero)
 * ``__is_unsigned`` (C++, Embarcadero):
   Returns false for enumeration types. Note, before Clang 13, returned true for
   enumeration types if the underlying type was unsigned.
 * ``__is_void`` (C++, Embarcadero)
 * ``__is_volatile`` (C++, Embarcadero)
 * ``__reference_binds_to_temporary(T, U)`` (Clang):  Determines whether a
   reference of type ``T`` bound to an expression of type ``U`` would bind to a