MC: Shrink MCDwarfLoc/MCLineEntry
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.)

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@241970 91177308-0d34-0410-b5e6-96231b3b80d8

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