[lld] [llvm] [Support] Support nested parallel TaskGroup via work-stealing (PR #189293)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The first commit is actually https://github.com/llvm/llvm-project/pull/189196 . Will rebase once 189196 lands.

https://github.com/llvm/llvm-project/pull/189293