Merge 2019-07 LWG Motion 21

P1065R2 Constexpr INVOKE

Fixes #3025.

Author: zygoloid

source/support.tex

@@ -582,6 +582,8 @@
 \defnlibxname{cpp_lib_constexpr}                            & \tcode{201811L} &
   any C++ library header from \tref{headers.cpp} or
   any C++ header for C library facilities from \tref{headers.cpp.c} \\ \rowsep
+\defnlibxname{cpp_lib_constexpr_invoke}                     & \tcode{201907L} &
+  \tcode{<functional>} \\ \rowsep
 \defnlibxname{cpp_lib_constexpr_swap_algorithms}            & \tcode{201806L} &
   \tcode{<algorithm>} \\ \rowsep
 \defnlibxname{cpp_lib_constexpr_dynamic_alloc}              & \tcode{201907L} &