DebugInfo: use strongly typed enum for debug info flags

Use ADT/BitmaskEnum for DINode::DIFlags for the following purposes:
    * Get rid of unsigned int for flags to avoid problems on platforms with sizeof(int) < 4
    * Flags are now strongly typed

Patch by: Victor Leschuk <vleschuk@gmail.com>

Differential Revision: https://reviews.llvm.org/D23766

llvm-svn: 280686