Extreme slowdowns when process memory usage is high

Whenever MSBuild completes a `BuildRequest`, it [checks](https://github.com/Microsoft/msbuild/blob/4e405a802d587368581117daf3402491beea8de7/src/XMakeBuildEngine/BackEnd/Components/BuildRequestEngine/BuildRequestEngine.cs#L771) to see if the current process is using "too much" memory (defined as 80% of the virtual memory limit). If it is, the engine [attempts to free memory](https://github.com/Microsoft/msbuild/blob/4e405a802d587368581117daf3402491beea8de7/src/XMakeBuildEngine/BackEnd/Components/BuildRequestEngine/BuildRequestEngine.cs#L854-L862) by flushing build results to a file-based cache and calling for garbage collection.

This system likely worked well at some point, when process memory usage came primarily from MSBuild and flushing results to disk released a significant amount of memory. But if the bulk of the memory usage is coming from something _else_ in the process, this can cause extreme slowdowns: every build request completion does synchronous I/O _and_ a stop-the-world GC. This is especially painful if the process in question is `devenv.exe` and it's in the midst of a solution-configuration change (i.e. "do a design-time build of every project in the solution").