[tsl:concurrency] Add CountDownAsyncValueRef to concurrency library #19403
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[tsl:concurrency] Add CountDownAsyncValueRef to concurrency library
This idiom is used in a few places in XLA:CPU executor, and TPU runtime.
Conceptually it's similar to https://en.cppreference.com/w/cpp/thread/latch, but for async values.
BM_CountDownSuccess/4 31.4 ns 31.4 ns 19308520
BM_CountDownSuccess/8 37.1 ns 37.0 ns 18911191
BM_CountDownSuccess/16 57.3 ns 57.3 ns 12371255
BM_CountDownSuccess/32 90.1 ns 90.0 ns 7746279
BM_CountDownError/4 75.8 ns 75.8 ns 8899579
BM_CountDownError/8 110 ns 110 ns 6349328
BM_CountDownError/16 171 ns 171 ns 4068219
BM_CountDownError/32 290 ns 290 ns 2402659