Update www_status/add test for P1972:

This seems to have been implemented/supported correctly back in Clang
10, so update the documentation and add the test from the paper.

Author: Erich Keane

clang/test/SemaCXX/cxx20-check-fptr-constraints.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+namespace P1972 {
+void f(int) requires false; // expected-note 4{{because 'false' evaluated to false}} \
+                            // expected-note{{constraints not satisfied}}
+void g() {
+  f(0);                      // expected-error{{no matching function for call to 'f'}}
+  void (*p1)(int) = f;       // expected-error{{invalid reference to function 'f': constraints not satisfied}}
+  void (*p21)(int) = &f;     // expected-error{{invalid reference to function 'f': constraints not satisfied}}
+  decltype(f) *p2 = nullptr; // expected-error{{invalid reference to function 'f': constraints not satisfied}}
+}
+}

clang/www/cxx_status.html

@@ -938,7 +938,7 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr> <!-- from Belfast -->
         <td><a href="https://wg21.link/p1972r0">P1972R0</a></td>
-        <td rowspan="4" class="none" align="center">No</td>
+        <td rowspan="4" class="full" align="center">Clang 10</td>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p1980r0">P1980R0</a></td>