[clang] [llvm] Allow Custom LTO Linking And Fix Warp Sizes (PR #224691)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> @shiltian the motivation is to handle line 8 in the "Codegen - Priority" part of the internal spreadsheet. We are trying to bring ROCm and upstream closer together. This plus the companion PR together help do that.

The problem with all of these out of tree patches is they 

1. Don't have tests
2. Aren't broken down appropriately
3. Don't necessary make sense

This PR covers wildly different areas without any new test coverage? This needs to be broken up and the test gaps filled 

https://github.com/llvm/llvm-project/pull/224691