Remove CleanupHackLevel from CGException.

This patch removes the dead code, and refines the
getEHResumeBlock() slightly.

The CleanupHackLevel was a hack to the old exception
handling intrinsics, which have several issues with function
inliner.

Since LLVM 3.0, the new landingpad and resume instructions
are added to LLVM IR.  With the new exception handling
mechanism, most of the issues are fixed now.  We should
always use these instructions to implement the exception
handling code nowadays, and we don't need the hack any more.

Besides, the `CleanupHackLevel` is a compile-time constant,
thus other cases have been considered as dead code for a while.