MC: Shrink MCDwarfLoc/MCLineEntry

Drop 8 bytes off of `MCDwarfLoc` by restricting the `Isa`, `Column`, and
`Flags` members to appropriate sizes (from `DWARFDebugLine::Row`).
Saves a little over 0.5% off the heap of llc with no real functionality
change.

(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)

llvm-svn: 241970