P1065R2 constexpr INVOKE

This needs compiler support for P0859R0 "CWG 1581: When are constexpr member functions defined?".

Feature-test macro as of WG21-N4842 (note that WG21-P1065 proposed adding __cpp_lib_constexpr_invoke, which was renamed by WG21-P1902):
#define __cpp_lib_constexpr_functional 201907L
(This was originally added for #50 WG21-P1032 "Miscellaneous constexpr" with the value 201811L, see SD-6.)