[version.syn] Revert __cpp_lib_chrono back to 201907L as directed by @brevzin in #3476.

Author: Dawn Perchik

papers/nxxxx.md

@@ -43,6 +43,16 @@ CWG motion 2: [P1234R5 "Paper name"](http://wg21.link/p1234r5), resolving 3 NB c
 
 ...
 
+### LWG motion 19
+
+Did not change the value of the `__cpp_lib_chrono` macro.
+
+The requested new value of this macro (`201803L`)
+is actually lower than the current value
+(`201907L`, not `201611L` as listed in [P1902R1](http://wg21.link/p1902r1)).
+The chair of SG10 has confirmed that the request to change this macro's value
+is an error. The pre-existing, higher value is retained.
+
 ## Disposition of editorial NB comments on C++ 2020 CD1
 
 Listed below are draft disposition for all comments that were

source/support.tex

@@ -574,7 +574,7 @@
 #define @\defnlibxname{cpp_lib_byte}@                              201603L // also in \libheader{cstddef}
 #define @\defnlibxname{cpp_lib_char8_t}@                           201907L
   // also in \libheader{atomic}, \libheader{filesystem}, \libheader{istream}, \libheader{limits}, \libheader{locale}, \libheader{ostream}, \libheader{string}, \libheader{string_view}
-#define @\defnlibxname{cpp_lib_chrono}@                            201803L // also in \libheader{chrono}
+#define @\defnlibxname{cpp_lib_chrono}@                            201907L // also in \libheader{chrono}
 #define @\defnlibxname{cpp_lib_chrono_udls}@                       201304L // also in \libheader{chrono}
 #define @\defnlibxname{cpp_lib_clamp}@                             201603L // also in \libheader{algorithm}
 #define @\defnlibxname{cpp_lib_complex_udls}@                      201309L // also in \libheader{complex}