patch: add 'nonnull' attribute when passing reference types

[Nick Lewycky]

The attached patch updates CodeGen to add the llvm 'nonnull' attribute
whenever the return or argument type is a reference (lvalue or rvalue
reference).

There is more we could do here (for instance, 'this' pointers are also
never null), but I'm starting here since the bulk of the change is merely
updating the tests. In particular I added 'nonnull' whenever the mangled
name indicated a reference type was involved. For Microsoft mangling tests
and some Objective-C++ tests, I simply updated the tests with no attempt to
reason about the correctness.

Please review!

Nick
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