[runtime] Complete support for exception marshalling on CoreCLR.

[rolfbjarne]

• This also required an empty implementation of the toggle ref machinery,
  because the ObjectiveCMarshal.Initialize call to initialize unhandled
  exception support requires passing toggle ref callbacks as well. The
  complete implementation is more complex, and will come in an uncoming PR.

• The TestConstrainedGenericType test can now be re-enabled, after a few
  updates.

[vs-mobiletools-engineering-service2]

❌ [PR Build] Tests failed on Build ❌

Tests failed on Build.

• Azure DevOps
• Html Report (VSDrops)

API diff

✅ API Diff from stable

View API diff

• iOS vsdrops gist
• macOS vsdrops gist
• macCat vsdrops gist
• macCatiOS gist
• tvOS vsdrops gist
• watchOS vsdrops gist

API & Generator diff

✅ API Diff (from PR only) (no change)
✅ Generator Diff (no change)

GitHub pages

Results can be found in the following github pages (it might take some time to publish):

• Test results
• API diff
• API & Generator diff

Test results

2 tests failed, 170 tests passed.

Failed tests

• monotouch-test/Mac [dotnet]/Debug (CoreCLR) [dotnet]: Failed (Test run failed.
  Tests run: 2444 Passed: 2355 Inconclusive: 7 Failed: 3 Ignored: 86)
• monotouch-test/Mac [dotnet]/Debug (CoreCLR, static registrar) [dotnet]: Failed (Test run failed.
  Tests run: 2441 Passed: 2355 Inconclusive: 7 Failed: 1 Ignored: 85)

Pipeline on Agent XAMBOT-1099.BigSur'
Merge 11df830 into 51c9364

[rolfbjarne]

Test failures are unrelated:

• monotouch-test/Mac [dotnet]/Debug (CoreCLR) [dotnet]: Fixed in [tests] Adjust MX8029_b and MX8033 tests to cope with slightly different exception message output for CoreCLR. #11732 and [ObjCRuntime] Unwrap TargetInvocationExceptions we get when invoking methods using reflection. #11733.
• monotouch-test/Mac [dotnet]/Debug (CoreCLR, static registrar) [dotnet]: Fixed in [ObjCRuntime] Unwrap TargetInvocationExceptions we get when invoking methods using reflection. #11733 and [tests] Adjust MX8029_b and MX8033 tests to cope with slightly different exception message output for CoreCLR. #11732.