Roll buildroot to switch to VS 2019

[stuartmorgan-g]

Picks up flutter/buildroot#392

Fixes flutter/flutter#48831

[stuartmorgan-g]

Notes to myself:

• Windows Android AOT needs goma disabled, as expected (until/unless we can move that to clang at some point too). Since it's not building much, my expectation is that the resulting slowdown won't be a significant problem.
• Windows Host - static_assert failed due to requirement 'std::is_trivially_constructible<std::atomic<std::mutex *>>::value' -- Check to see if this is still an issue with latest Angle; the type of the mutex changed in Februrary.
• Windows Web - same assert failure

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite Windows Host Engine has failed. Please fix the issues identified (or deflake) before re-applying this label.

[stuartmorgan-g]

Windows host unit tests keep timing out. Trying a resync to see if it was an unrelated issue, but it's possible that VS 2019 is breaking unit tests somehow.

[stuartmorgan-g]

It's definitely breaking tests, so the buildroot change is being rolled back again. Based on recent experience with this bot, this may not actually be a timeout; C++ exceptions in unit tests on that bot cause this behavior too, without logging the exception, so that may well be what's happening. The fact that's unopt-only is consistent with that theory.

Unfortunately I'm not able to repro locally so far, even though it's consistent on CI.

[chinmaygarde]

Can we close this PR for now and attempt this at a later time if you don't have the cycles to work on it ATM?

[stuartmorgan-g]

I'm still working on in intermittently. I'll convert to draft though since it's clearly not ready as-is.

[stuartmorgan-g]

/cc @cbracken

[Hixie]

@stuartmorgan I'm assuming your work on this stalled, so I'm going to close the PR, but please don't hesitate to open a new one when you're ready. Thanks!