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

[Greg Clayton via llvm-commits]

clayborg wrote:

The change was introduced with this patch:
```
commit e79dda31e9ed6cfc97d06ad87976fb16e53f3780
Author: David Blaikie <dblaikie at gmail.com>
Date:   Tue Sep 19 18:36:11 2017 +0000

    dwarfdump/symbolizer: Avoid loading unneeded CUs from a DWP
    
    When symbolizing large binaries, parsing every CU in a DWP file is a
    significant performance penalty. Instead, use the index to only load the
    CUs that are needed.
    
    llvm-svn: 313659
```

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