[libc++] [C++20] [P0415] Constexpr for std::complex.

This patch adds constexpr to <complex> header: operators, member operators, and member functions (real, imag, norm, conj).

https://eel.is/c++draft/complex.numbers
https://wg21.link/p0415

Reviewed By: ldionne, #libc

Spies: philnik, danilaml, Quuxplusone, wmaxey, arichardson, libcxx-commits

Differential Revision: https://reviews.llvm.org/D79555

NOKEYCHECK=True
GitOrigin-RevId: 7223bcf04c4ce3c1585df9408c6a6663cc8364ea
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index c4ecbd6..34e28a3 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -202,7 +202,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_algorithms``                ``201806L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_complex``                   *unimplemented*
+    ``__cpp_lib_constexpr_complex``                   ``201711L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_dynamic_alloc``             ``201907L``
     ------------------------------------------------- -----------------