[llvm] Make DWARFUnitVector threadsafe. (PR #71487)

[Alexander Yermolovich via llvm-commits]

ayermolo wrote:

Would be interesting to figure out what that history is.
On bolt side it will simplify some things.
Also that vector ends up being populated one at a time if whatever tool uses DWARFContext invokes getNonSkeltonUnitDIE through parseDWO-->getDWOCompileUnitForHash.

https://github.com/llvm/llvm-project/pull/71487