Steps to reproduce:
1. Open attached project
2. Open "Test" Scene
3. Enter Play mode
4. Select "TrimExcess-3.5" in the hierarchy
5. Toggle Disable/Enable checkmark for TestHashSetTrimExcessBug Script component

Expected result: the function trims the HashSet to the minimum size required to contain it.
Actual result: the HashSet increases in size each time TrimExcess is called on it.

Reproduced with: 5.6.1f1, 5.6.1f1, 2017.3.1f1, 2017.3.1f1, 2018.2.0a1

Note: the issue does not appear when using .NET 4.6. It can be tested by enabling "TrimExcess-4.6" game object in the hierarchy and repeating step #5 with it.