Establish the <bit> header. NFC yet. Reviewed as https://reviews.llvm.org/D50815

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@339943 91177308-0d34-0410-b5e6-96231b3b80d8

include/module.modulemap

@@ -228,6 +228,10 @@ module std [system] {
     header "atomic"
     export *
   }
+  module bit {
+    header "bit"
+    export *
+  }
   module bitset {
     header "bitset"
     export string