[libc++] Make __decay_copy constexpr

This is going to be necessary to implement some range adaptors.
As a fly-by fix, rename _LIBCPP_INLINE_VISIBILITY to _LIBCPP_HIDE_FROM_ABI
and remove a redundant inline keyword.

Differential Revision: https://reviews.llvm.org/D112650

Author: ldionne

libcxx/include/__utility/decay_copy.h

@@ -21,9 +21,10 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY typename decay<_Tp>::type __decay_copy(_Tp&& __t)
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+typename decay<_Tp>::type __decay_copy(_Tp&& __t)
 #if _LIBCPP_STD_VER > 17
-    noexcept(is_nothrow_convertible_v<_Tp, remove_reference_t<_Tp> >)
+    noexcept(is_nothrow_convertible_v<_Tp, remove_reference_t<_Tp>>)
 #endif
 {
   return _VSTD::forward<_Tp>(__t);