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

[Kevin Frei via llvm-commits]

kevinfrei wrote:

> OK. I think this two lock thing makes sense in my head. One concern I have is that between locks in tryExtractDIEsIfNeeded it switches to clearDIEs.

Oooh! You're right: There's a spot in there where bad things could happen. I'll get that fixed.

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