[llvm] Reduce llvm-gsymutil memory usage (PR #91023)

[Kevin Frei via llvm-commits]

kevinfrei wrote:

ping @ayermolo @dwblaikie This is blocking some dwarf-health tracking work I'm trying to make headway on. The only 'stylistic' adjustment I can see making sense is to have some sort of doubled-checked locking abstraction that takes a lock and a couple lambda's to make the duplicated code cleaner, which I'm happy to do if someone wants it, but I haven't seen something like that elsewhere.

https://github.com/llvm/llvm-project/pull/91023