Fixes for various issues

[yaakov-h]

Fixes for:

• .NET 4.6.1 in VS2015, .NET 4.6 and 4.6.1 in VS2013 (also PR Use .NET 4.6 reference assemblies for 4.6.1 #359, fixes string.IsNullOrWhiteSpace() not considered Pure by Requires() or Invariant() when running on .NET 4.6.1 #339)
• Add missing postcondition to System.Type.GetMethods(BindingFlags). (fixes BCL: System.Type.GetMethods() is missing an ensures #414)
• Remove invalid precondition on System.Linq.Expression.Expression.NewArrayInit (fixes Static checker cannot prove compiler-generated Expressions are created safely. #424)
• Disable static checking on ContractDeclarativeAssemblyAttribute in an attempt to improve performance (see cccheck: Cached analysis gets horrendously slow over time #423).

[SergeyTeplyakov]

It seems that there is a bunch of commits for this fix:), but everything looks good to me.

If all the tests will pass, I'll merge it.

[yaakov-h]

Tests.RewriterTest.BuildRewriteRunFromSourcesRoslynV45 [SKIP]
Old Roslyn bits are not compatible with CCRewrite. Test (and old binaries) should be removed in the next iteration."

Heh.

[SergeyTeplyakov]

@yaakov-h Yeah... I don't event sure what exactly this options.IsLegacyRoslyn = true; means. Because another test (next to skipped one) is still enabled. So this comment doesn't means that roslyn compiler ignored.