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

[Shilei Tian via cfe-commits]

shiltian wrote:

I mean, I understand the part about bridging the gap between upstream and downstream, but I don't understand the motivation for doing this PR downstream in the first place, such as what problem we're trying to solve and why we need a new LTO phase for this.

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