[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:

could it make sense to have this class also as an interface / abstract class, or with virtual methods to allow them to be overridden?

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