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

[David Blaikie via llvm-commits]

dwblaikie wrote:

Ah, I thought this was all in gsym, so hadn't given the review much attention.

Are these independent changes - the line table clearing stuff presumably is independent, for instance? I guess the unit DIE  clearing is dependent on the extra locking being added?

The extra locking is unfortunate/more difficult to just straight up approve, as we'd previously restricted the multithreading support to the DWARFContext by having distinct contexts - so that non-multithreaded code didn't take the hit or complexity for the multithreaded code. But this new codepath and locking is present for single and multithreaded uses alike, by the looks of it? Not sure what to do about that... 

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