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

[Greg Clayton via llvm-commits]

clayborg wrote:

> Can Lazy argument be removed from &getDWOUnits(bool Lazy), since we always now pass false to underlying call.

We don't always pass false. Only in the threadsafe code do we pass false. In the single threaded code we still pass true for Lazy since it is ok.

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