Move net451 test assets to net461

[jaredpar]

This migrates our tests which depended on net451 assemblies to use net461 assemblies instead. The reason for this is to consolidate our test assets to make them easier to manager. This also saves 127MB of disk I/O when doing a restore in CI which should reduce PR validation time. The under pinning mentality of this change is that net451 and net461 are not significantly different hence this update should be very straight forward.

This PR is broken into two commits. The first commit is a mostly mechanical migration from net451 to net461. There are a number of places where I had to do minor fixups in baselines like changing System.dll(net451) to System(net461). The only significant functional issue that popped up reading through the tests is a number of net451 tests depended on Array.Empty<T> not existing. Those required functional changes to work: typically by using MakeMemberMissing calls to maintain the behavior. To help reviewing these are the tests impacted by this behavior.

• CodeGenExprLambdaTests.MethodCallWithParams
• CollectionExpressionTests.ArrayEmpty_01
• CollectionExpressionTests.ArrayEmpty_02
• InternalsVisibleToAndStrongNameTests.IvtVirtualCall1
• InternalsVisibleToAndStrongNameTests.IvtVirtualCall2
• MissingSpecialMember.AllSpecialTypeMembers

Note: Had to temporarily disable BC30645ERR_InvalidOptionalParameterUsage1b. Will re-enable that test once I can publish to nuget.org again, fix the ref assembly bug that will let me port this test to net461

The second commit is refactor rename of various APIs that had the partWithMscorlib45 to WithMscorlib461. Suggest mostly spot checking that one.

[jaredpar]

@dotnet/roslyn-compiler PTAL

[333fred]

I spot checked a number of the files in the PR, and went through the test helper classes. Going to sign off overall.

[jaredpar]

Integration test failures are unrelated