[llvm] [lldb] DEBUGINFOD based DWP acquisition for LLDB (PR #70996)

[Nico Weber via llvm-commits]

nico wrote:

This breaks targeting macOS versions older than 10.12 for unnecessary reason:

```
../../llvm/lib/Debuginfod/Debuginfod.cpp:54:6: error: 'shared_mutex' is unavailable: introduced in macOS 10.12
std::shared_mutex UrlsMutex;
     ^
```

You should be able to use llvm::sys::RWMutex so please switch to that. See https://reviews.llvm.org/D138423 for an example.

https://github.com/llvm/llvm-project/pull/70996