[libc++] Move fpos into its own header

For some reason `<string>` defines `std::fpos`, which should be defined in `<ios>`.

Reviewed By: Quuxplusone, Mordante, #libc

Spies: libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D118914

NOKEYCHECK=True
GitOrigin-RevId: 679f8a885b65e0c82537590f1d88c2bc4c6869bc
diff --git a/include/module.modulemap b/include/module.modulemap
index 0e2e761..35c4777 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -575,6 +575,10 @@
     header "ios"
     export iosfwd
     export *
+
+    module __ios {
+      module fpos { private header "__ios/fpos.h" }
+    }
   }
   module iosfwd {
     header "iosfwd"