[libc++] Modularize <chrono>

I didn't split the calendar bits more than this because there was little
benefit to doing it, and I know our calendar support is incomplete.
Whoever picks up the missing calendar bits can organize these headers
at their leisure.

Differential Revision: https://reviews.llvm.org/D116965

NOKEYCHECK=True
GitOrigin-RevId: 5726e55981044b3b2c5bc98272e9ccd0bf58dbec
diff --git a/include/module.modulemap b/include/module.modulemap
index e7f197e..33e0ecf 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -364,6 +364,16 @@
   module chrono {
     header "chrono"
     export *
+
+    module __chrono {
+      module calendar              { private header "__chrono/calendar.h" }
+      module duration              { private header "__chrono/duration.h" }
+      module file_clock            { private header "__chrono/file_clock.h" }
+      module high_resolution_clock { private header "__chrono/high_resolution_clock.h" }
+      module steady_clock          { private header "__chrono/steady_clock.h" }
+      module system_clock          { private header "__chrono/system_clock.h" }
+      module time_point            { private header "__chrono/time_point.h" }
+    }
   }
   module codecvt {
     header "codecvt"