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

[Kevin Frei via llvm-commits]

kevinfrei wrote:

To alleviate copy-pasta problems, I've added a local DoubleCheckewhatdRWLocker helper function that takes the "check if it's already been initialized" lambda, and the initializer lambda (and the lock). I feel like the result is a much cleaner version of the original diff, plus it has the third lock to prevent free's from occurring between the single CU and the array CU initialization code that @ayermolo saw. @dwblaikie they tell me you're the guy who needs to approve this, now...

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