AsmPrinter: Avoid creating symbols in DwarfStringPool

Stop creating symbols we don't need in `DwarfStringPool`.  The consumers
only call `DwarfStringPoolEntryRef::getSymbol()` when DWARF is
relocatable, so this just stops creating the unused symbols when it's
not.  This drops memory usage from 851 MB to 845 MB, around 0.7%.

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

llvm-svn: 238122