Merge 2019-07 LWG Motion 4

P1424R1 'constexpr' feature macro concerns

Fixes #3008.

Author: zygoloid

source/support.tex

@@ -571,9 +571,9 @@
   \tcode{<complex>} \\ \rowsep
 \defnlibxname{cpp_lib_concepts}                             & \tcode{201806L} &
   \tcode{<concepts>} \\ \rowsep
-\defnlibxname{cpp_lib_constexpr_misc}                       & \tcode{201811L} &
-  \tcode{<array>} \tcode{<functional>} \tcode{<iterator>}
-  \tcode{<string_view>} \tcode{<tuple>} \tcode{<utility>} \\ \rowsep
+\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_swap_algorithms}            & \tcode{201806L} &
   \tcode{<algorithm>} \\ \rowsep
 \defnlibxname{cpp_lib_constexpr_dynamic_alloc}              & \tcode{201907L} &