[AsmParser] Add missing globals declarations in incomplete IR mode (#79855)

If `-allow-incomplete-ir` is enabled, automatically insert declarations
for missing globals.

If a global is only used in calls with the same function type, insert a
function declaration with that type.

Otherwise, insert a dummy i8 global. The fallback case could be extended
with various heuristics (e.g. we could look at load/store types), but
I've chosen to keep it simple for now, because I'm unsure to what degree
this would really useful without more experience. I expect that in most
cases the declaration type doesn't really matter (note that the type of
an external global specifies a *minimum* size only, not a precise size).

This is a followup to https://github.com/llvm/llvm-project/pull/78421.

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