DebugInfo: Correct the location of EH cleanup for blocks

This was previously piggybacking on whatever happened to be the last
location set on CGDebugInfo/DIBuilder, which was wrong (it was often the
current location, such as the 'fn()' call site, not the end of the
block). With my improvements to set/unset the location in a scoped
manner (r225000) this went from a bad quality situation, to a crash.
Fixing this goes part-way to unblocking the recommit of r225000.

It's likely that any call to CodeGenFunction::StartFunction without the
CurEHLocation set represents a similar bug or risk of a bug. Perhaps
there are some callers that know they won't generate EH cleanups, but
I'm not sure.

I considered a generic catch-fix in StartFunction (just fallback to the
GlobalDecl's location) but that seemed like it'd mask bugs where the EH
location shouldn't be the same as the decl's location (& indeed by not
using that stop-gap I found this bug). We'll see how long I can hold out
on the generic catch-all. I might eventually be able to add an assertion
in.

llvm-svn: 225845