[dmd-internals] Walter -- Please apply the patch in bug 5447

[Don Clugston]

5447 Should be illegal to throw a non-Throwable

This is necessary to implement exception chaining. Once it is in
place, I can check in my druntime changes and we'll have
nearly-complete
exception handling on Windows.

Please also apply this trivial D1-only critical patch, which has 6 votes:
5015 Regression(1.061): Cyclic import breaks is() in a static if