[llvm] Modify lit config to allow specify extra env vars to pass through to lit invocation (PR #208239)

[David Young via llvm-commits]

youngd007 wrote:

As an example, there are actually LLDB settings that are set/read by an env var:
https://github.com/llvm/llvm-project/blob/48aade5e5208414693b695b1dfdb6c95ae56ccf6/llvm/lib/Debuginfod/Debuginfod.cpp#L99
So setting a specific value for a test could definitely be a thing that would need to pass to lit presumably.

https://github.com/llvm/llvm-project/pull/208239