Allow libc++ to use modules in C++03.

Libc++ internal uses <atomic> in C++03 code but the module map forbids its use.
This causes the libc++ 'std' module to fail to build in C++03.

This patch removes the requirement to fix this issue.

llvm-svn: 287693

libcxx/include/module.modulemap

@@ -213,7 +213,6 @@ module std [system] {
   module atomic {
     header "atomic"
     export *
-    requires cplusplus11
   }
   module bitset {
     header "bitset"