Fix a pretty nasty bug in noreturn destructors that cascaded into lots

of false positive warnings that depend on noreturn destructors pruning
the CFGs, but only in C++0x mode!

This was really surprising as the debugger quickly reveals that the
attributes are parsed correctly (and using the same code) in both modes.
The warning fires in the same way in both modes. But between parsing and
building the destructor declaration with the noreturn attribute and the
warning, it magically disappears. The key? The 'noexcept' appears!

When we were rebuilding the destructor type with the computed implicit
noexcept we completely dropped the old type on the floor. This almost
makes sense (as the arguments and return type to a destructor aren't
exactly unpredictable), but lost any function type attributes as well.
The fix is simple, we build the new type off of the old one rather than
starting fresh.

Testing this is a bit awkward. I've done it by running the
noreturn-sensitive tests in both modes, which previous failed and now
passes, but if anyone has ideas about how to more specifically and
thoroughly test that the extended info on a destructor is preserved when
adding noexcept, I'm all ears.

llvm-svn: 140138