LLVM: implement debug info for structs

This involved some significant reworking in order to introduce the
concept of "forward declarations" to the system to break dependency
loops.

The `lowerDebugType` function now takes an `enum { full, fwd }` and is moved
from `DeclGen` to `Object` so that it can be called from `flushModule`.

`DITypeMap` is now an `ArrayHashMap` instead of a `HashMap` so that we can
iterate over the entries in `flushModule` and finalize the forward decl
DITypes into full DITypes.

`DITypeMap` now stores `AnnotatedDITypePtr` values instead of
`*DIType` values. This is an abstraction around a `usize` which assumes
the pointers will be at least 2 bytes aligned and uses the least
significant bit to store whether it is forward decl or a fully resolved
debug info type.

`lowerDebugTypeImpl` is extracted out from `lowerDebugType` and it has a
mechanism for completing a forward decl DIType to a fully resolved one.
The function now contains lowering for struct types. Closes #11095.

There is a workaround for struct types which have not had
`resolveFieldTypes` called in Sema, even by the time `flushModule` is
called. This is a deficiency of Sema that should be addressed, and the
workaround removed. I think Sema needs a new mechanism to queue up type
resolution work instead of doing it in-line, so that it does not cause
false dependency loops. We already have one failing behavior test
because of a false dependency loop.

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