Add operators to make launch a bitmask type.  Searched all of the standard, and libc++ to see if this error occurred elsewhere and didn't see any other place.  This fixes http://llvm.org/bugs/show_bug.cgi?id=16207

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185265 91177308-0d34-0410-b5e6-96231b3b80d8