I also added `// skip-codegen` to each one, to address potential concerns
that this change would otherwise slow down our test suite spending time
generating code for files that are really just meant to be checks of
compiler diagnostics.

(However, I will say: My preference is to not use `// skip-codegen` if
one can avoid it. We can use all the testing of how we drive LLVM that
we can get...)

(Updated post rebase.)

The error message is sub-par, but fixing that requries moving ScalarMaybeUndef
to librustc which would conflict badly with another PR that is in flight.

Since the enums get optimized down to 1 byte long, the bits
set in the usize member don't align with the enums on big-endian
machines. Avoid this issue by shrinking the integer member to the
same size as the enums.

This gives at least some explanation for why a borrow is expected to
last for a certain free region. Also:

* Reports E0373: "closure may outlive the current function" with NLL.
* Special cases the case of returning a reference to (or value
  referencing) a local variable or temporary (E0515).
* Special case assigning a reference to a local variable in a closure
  to a captured variable.