Mark Permissions as a bitmask enum

this allows one to use bitwise operators on the variables of this type
without complicated casting.

The gotcha here is that the combinations of these enums were being used
in some constexpr contexts such as case labels (case
ePermissionsWritable | ePermissionsExecutable:), which is not possible
if the user-defined operator| is not constexpr.

So, this commit also marks these operators as constexpr. I am committing
this as a small standalone patch so it can be easily reverted if some
compiler has an issue with this.

llvm-svn: 349149