AsmPrinter: Avoid EmitLabelDifference() in DwarfAccelTable

Mint a new function, `AsmPrinter::emitDwarfStringOffset()`, which takes
a `DwarfStringPoolEntryRef`.  When DWARF is relocatable across sections,
this defers to `emitSectionOffset()` and emits the `MCSymbol`;
otherwise, just emit the offset directly, without using any intermediate
symbols.

`EmitLabelDifference()` is already optimized to emit absolute label
differences cheaply when possible, so there aren't any major memory
savings here (853 MB down to 851 MB, or 0.2%).  However, it prepares for
making the `MCSymbol`s in the `DwarfStringPool` optional.

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

llvm-svn: 238119