[libcxx] Remove CPlusPlus11 requirements from the module.modulemap

[EricWF]

Summary

This patch does two things:

• Allow std.strstream to used in C++11. This is needed to get the libc++ test suite passing with modules enabled. Also enforcing that strstream is not used in C++11 seems like non-conforming behavior.

• Remove the C++11 requirement for std.atomic. Libc++ implements almost all of it in C++03.

include/module.modulemap

module std [system] {
   module array {
     header "array"
     export initializer_list
     export *
   }
   module atomic {
     header "atomic"
     export *
-    requires cplusplus11
   }
   module bitset {
     header "bitset"
     export string
     export iosfwd
     export *
   }
   // No submodule for cassert. It fundamentally needs repeated, textual inclusion.
module std [system] {
   }
   module string {
     header "string"
     export initializer_list
     export *
   }
   module strstream {
     header "strstream"
-    requires !cplusplus11
   }
   module system_error {
     header "system_error"
     export *
   }
   module thread {
     header "thread"
     export *