EH: Add beginfinally intrinsic

[JosephTremoulet]

Intrinsic definition and lowering (to nothing; intrinsic is just to represent sideffects of outer filters).

Part of #13.

[JosephTremoulet]

We won't need an intrinsic for this after all; since exceptions can only be raised by external calls, the sideffects of invoke will already include potentially executing the outlined, address-escaped filter function. This is how SEH filter invocations are modeled (see http://article.gmane.org/gmane.comp.compilers.llvm.devel/84618).