[llvm] [mlir] Split the llvm::ThreadPool into an abstract base class and an implementation (PR #82094)

[via llvm-commits]

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@@ -227,7 +265,7 @@ class ThreadPool {
 class ThreadPoolTaskGroup {
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fabrizio-indirli wrote:

I see; personally I wouldn't need to customize this class at this time, thus for me it could be fine to leave it as it is. 
But parhaps in general there could be some use cases where one would want to customize the async(), wait() and destructor methods for a group? E.g. keeping track of the num of tasks in the group and refusing to enqueue a task if the group is already too busy.

https://github.com/llvm/llvm-project/pull/82094