Do not check IsEmpty for every item in ConcurrentBag.Clear

[SimonCropp]

The ConcurrentBag<T>.Clear polyfill (.NET Framework / netstandard2.0) checked IsEmpty before every TryTake. On .NET Framework IsEmpty freezes the bag under its global lock, so it costs more than the TryTake it guards.

TryTake already returns false once the bag is empty, so the loop is now:

while (target.TryTake(out _))
{
}

Same result, including when other threads add or take concurrently: both versions stop the first time the bag is observed empty.

Measurements

net48, Release, x64, Stopwatch around Clear only, best of 7, microseconds per call:

Items	Before	After
0	0.09	0.24
10	1.23	1.01
1,000	82.3	37.0
100,000	7,281	3,032

Clearing an already empty bag is about 0.15 us slower, because a failing TryTake (which tries to steal from other threads' lists) costs more than IsEmpty. Guarding with one IsEmpty up front would fix that case and make every other case pay for it, so I left it out.

Tests

Added to PolyfillTests_ConcurrentBag.cs: clearing bags of 0, 1, 2 and 1,000 items (IsEmpty, Count, ToArray, TryTake, TryPeek afterwards, clearing twice, and reuse after clearing); a bag filled from four threads, so that items live in several thread-local lists; null and default items.