[libc++] Put __memory headers in submodules

[teemperor]

Summary

Currently all the headers in __memory are not submodules of std but normal headers that get duplicated everywhere.

This patch marks all of them that can be parsed on their own as submodules so we only need one copy of them around.

This also means Clang no longer has to deal with making sense of all the duplicated declarations which leads to bogus
module build failures as seen in D103369.

[teemperor]

That should fix the build failures that caused the revert of D103369.

[• Quuxplusone]

I believe this is effectively a subset of @cjdb's D103551, and D103551 will be the better (completer) fix, so this one should be abandoned.

[teemperor]

You are right, I wasn't aware of D103551. Thanks!

Abandoning this in favour of D103551

libcxx/include/module.modulemap

module std [system] {
   module __node_handle { header "__node_handle" export * }
   module __split_buffer { header "__split_buffer" export * }
   module __std_stream { header "__std_stream" export * }
   module __string { header "__string" export * }
   module __threading_support { header "__threading_support" export * }
   module __tree { header "__tree" export * }
   module __tuple { header "__tuple" export * }
   module __undef_macros { header "__undef_macros" export * }
+  module __memory {
+    // These can only be parsed when 'allocator' was defined.
+    textual header "__memory/allocator_traits.h"
+    textual header "__memory/allocation_guard.h"
+    umbrella "__memory"
+    export *
+  }
 
   module experimental {
     requires cplusplus11
 
     module algorithm {
       header "experimental/algorithm"
       export *
     }