[llvm] Reduce llvm-gsymutil memory usage (PR #139907)

[David Blaikie via llvm-commits]

https://github.com/dwblaikie approved this pull request.

`doWorkThreadSafely` feels like a bit of a big/generic hammer to wedge into the generic interface, when the goal was for that interface to not know/care about thread safety... But given the various callers into DwarfUnit trying to parse DIEs, not sure there's much more to it (short of defining thread safe DwarfUnit wrappers, etc) 

Guess this'll go on the tech debt ledger for a future dwarf parser rewrite.. 

https://github.com/llvm/llvm-project/pull/139907