[PATCH] D27155: Merge strings using concurrent hash map (3rd try!)

[Rafael Avila de Espindola via llvm-commits]

> Last but not least, I would still suggest checking how many strings .dwo
> avoids copying.

I did two builds of clang, and with -gsplit-dwarf and one without. I
linked both with gold to be sure the .dwo case is handled correctly.

In the non-dwo case .debug_str is 108_981_580 bytes long. On the dwo
case it is 468_352 bytes long.

Cheers,
Rafael